
I think pretty much everyone knows that when installing new fittings on a cored boat we should first remove the core before filling the void with thickened epoxy and then re-drilling for the fastener(s).
There are a bunch of different ways to do this, but the set of Alfa Tools Forstner bits I just bought myself, while far from inexpensive, are the quickest, neatest, and generally best way I have found to make a suitable hole in the outer skin, and remove most of the core, without penetrating the inner skin.
Way better and more controllable than a hole saw.
Highly recommended.