I have a couple of tape measures (one metric, the other imperial) in my tool bag, and even have a really nice digital calliper for when dead-nuts measurement is required, but the two measurement tools in my kit that get the most use are a cheap pocket calliper (top photo) and an even cheaper metal rule (below) .
Both are dual metric and imperial.
I recommend both for every cruiser’s tool bag. Buy two of each…they don’t float.
Lots more on maintaining a cruising boat.