The Offshore Voyaging Reference Site

Don’t Swim Near Marinas

A couple of weeks ago we were alongside at a marina in the Bras d’Or Lakes, Nova Scotia and I needed to take a look at the prop.

The marina is a small one and the water is pretty clean. Also, we were on the fuel dock well away from the finger piers and other boats. Given these factors and that I knew the dive was only going to take a minute or two, I was tempted to just get it done before heading out on an overnight passage.

But instead, we motored out to a cove and anchored so I could get in the water, thereby delaying our departure.

Why would I go to all this trouble? Well, the Bras d’Or Lakes, while open to the sea at the northern end, are brackish, and we were about as far from the opening as you can get, so the water was probably near fresh.

And I have known for years that by swimming in fresh water around plugged-in boats (or even wharves with poorly-installed electrical connections on them), we take a very real risk of being killed by stray current which will run through our bodies since we have a lower resistance than fresh water.

(If you want to really understand that last sentence, our chapter on Ohms Law will help.)

Is Salt Water Safe?

However, I had always thought that this risk was negligible in salt water since its resistance is much lower than fresh water so the current (amperage) runs through the water, not us.

But AAC-friend-in-the-comments and boat-maintenance-guru Steve D’Antonio has written a convincing article questioning that assumption.

Now I have never heard of a shock fatality in sea water, but do I want to find out the hard way that Steve is right? Obviously not.

And anyway, as Steve points out, can we know the water is salty enough to be safe in an enclosed area where there may be a river, stream, or spring close by? Obviously not, particularly since fresh water can collect in a layer on top of sea water.


  • Don’t swim near marinas, wharves or plugged-in boats.
  • Do read Steve’s excellent article (scroll down a bit after clicking on the link) for a deeper understanding of this very real danger.
  • Think about adding an Equipment Leakage Circuit Interrupter as Steve recommends, if your boat is not so fitted (and most older boats are not). I’m going to look at least look into this over the coming winter, even though we have an isolation transformer, which goes a long way to reduce all kinds of electrocution risk.

One More Thought

Phyllis and I have long worn lifejackets whenever we are underway and on deck. (Our general reasoning is a subject for another day.)

But given the above, we will make doubly sure to be wearing them when docking since, if we fell in and there was stray electricity in the water the jacket could easily save our lives. As I understand it, the shock itself is often not what kills but rather drowning after being paralyzed by the leaking electrical current.


Please read Steve’s article (link above) before commenting or asking a question. He does a great job of explaining how the risks work and we don’t want to end up duplicating that here.

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Sam Shafer

As a diver who does most of his diving in marinas and around docks it is something I worry about. I dive dry with only head and hands exposed to the water, don’t know if this is any better or worse. The best solution is to have the marina cut the power(only happens when I am actually inspecting underwater power cables). Getting the marina to do that so you can dive and recover an item that someone has dropped is next to impossible. I have felt the tingle several times and have all ways left the area and reported it to the marina. The response from marina management has ranged from “well that is why you don’t swim in the marina” to actually tracking down and fixing the problem. The worst are the private docks I have been shocked more times around those than in marinas.(I have a strong theory for why this is) In the end it is the divers choice to take the job or not. You get paid the money and you take your chances.

Sam Shafer

Most of my marina repair work and recovery work has been in fresh water. I dive dry because the mountain lakes of East Tennessee, South Holston and Watagua are deep and cold, even in the summer, this summer in Watagua: 60 degrees F at 40′ and below 75′ the temp never changes 39-41 degrees F. Good to know that the dry suit helps protect against the electric shock as well.

Dick Stevenson

Hi John,
A good reminder!
If only there was a way to know one is safe to enter the water near your boat while at a marina/dock where there is AC power. Sometimes it can feel compelling to enter the water, but, as you correctly point out, seamanship is often doing the hard thing.
Having not used my 120v/60Hz AC shore power system in over a decade, I have done some looking around at any upgrades that might warranted and am also looking into installing an ELCI (similarly stimulated by Steve’s article cited above). Those of us with older boats might be interested in knowing that galvanic isolators have had design improvements that might make it wise to swap out old for new.
My best, Dick Stevenson, s/v Alchemy


Avoid derilict Marinas worldwide. Especially those maintained by any gov entity.
If you feel the need to accuse your slipmate of stealing your shaft zinc. Find a new home.


I change my zincs myself. I always disconnect shorepower before doing this and have someone watching me. My boat is in salt water …I am still here to report this….BTW excellent articles (yours and d’Antonio) like always. Thanks

Dick Stevenson

Hi Marek,
I believe the experts in this area would say that it is other people’s boats and the marina’s wiring that you need to be worried about as well as your own.
One of the problems with warnings about this problem is that most people get away with it. It is rare and un-predictable and one sees people in the water around marinas with some regularity. But one needs to hold onto the down side: if you do encounter bad luck, you will not kick yourself for ending up with a bad burn or something—instead, you may be dead and, for sure, this scenario does kill a predictable number of people each year.
I also change my zincs in the water, but I always find a nice anchorage to do so. It is far safer and, visibility is usually better and you will likely find it far more pleasant as, I have rarely encountered marina water I wished to enter.
My best, Dick Stevenson, s/v Alchemy


The point of my reply was that I WILL change my zinc changing habits accordingly…but I have to say that I have one zinc (teardrop with 2 smallish screws) which will be difficult to change if the boat is moving, even a little .

Dick Stevenson

Hi Marek,
Understood. If the zinc lends itself to it, sometimes it is possible to pre-position (and keep them secure) the screws with a dab of silicone. Dick

Matt Marsh

I concur 100% with John’s bottom-line recommendations on this issue.

It’s really important to remember that, with the exception of GFCIs, most circuit protection equipment in boats & marinas is meant to prevent overheating and fires – not to protect humans from shock.

It does not take much AC current to paralyze a person. You will definitely feel 5 mA, and 20 mA is enough to halt your breathing and make most muscles freeze up.

The question is what electric field strength in the water is necessary to make that much current flow through a swimmer. Modelling this is, not surprisingly, rather complicated. But, as a pretty good first approximation for a person in brackish to salty water:

  • 3 volts per metre will drive about 10 mA through you and cause loss of muscle control in the hands and feet
  • 6 V/m is enough to drive the 20 mA needed to paralyze the respiratory muscles, at which point you’re basically doomed unless someone pulls you out.

The electric fields in question are highly non-uniform. However, you can get a ballpark first-order approximation by just dividing the maximum AC voltage used on board by the distance to the nearest thing that’s electrically bonded to the AC source, usually the steel pilings or pontoons of the marina. For a 240 volt system, that yields a danger zone of approximately 80 metres, within which the electric field gradient might exceed the 3 V/m threshold. (A boat with an ELCI will have killed power long before this point, but you should never bet your safety on that.)

The situation’s worse in fresh water, because your body’s conductivity is high relative to that of the water, and the electric fields change their shape and intensity accordingly. This does not mean that it’s safe to swim between boats and pilings in salt water, only that you need to allow a larger margin between power sources and ground paths when in fresh water.

(Here’s a little more from NIH on this issue, for those who like a bit of medical science: )


Can´t you assume that if there are fish in the water with a normal behaviour you are safe?.
I always thought that an electric field in the water dangerous for a human have to affect to the fish too. Am I in a mistake?.


Electrofishing is a thing. You hang an electrode off a pole, and any fish between that electrode and the metal hull of the boat (or a ground plate) get stunned. As a rule of thumb, it takes about 0.1 milliwatt of electric power per gram of body mass to knock a fish unconscious. (You don’t see electrofishing in saltwater much, because the much higher conductivity of the water means you waste an insane amount of power in the water around the fish in order to get enough current flowing through the fish.)

But yeah, don’t assume that because the fish aren’t being zapped, you won’t be zapped. You’re much bigger and your physiology is very different.

Stephen Lewinton


Really interesting article. Does the stray current phenomena impact on galvanic corrosion rates for aluminium and metal boats. If so how does one protect against it and/or detect the risk in a particular berth?

Really enjoy the site and as an inexperienced sailor lots to learn.



Steven D'Antonio

Many people ask about some type of warning device that would alert you to stray AC current before swimming. The problem with warning devices is the fault can occur at any moment, or it can be intermittent. Thus, you could be in water when your neighbor’s faulty HVAC unit kick on, and you are done for.

It’s important to note that this sort of stray AC current is typically not a corrosion issue, corrosion is, for the most part, a DC phenomenon.