This is my other favorite bit the Dremel #654. I like this one because most hardware, like stanchions, on 28-42 foot sailboats, is through bolted with 1/4" X 20 thread bolts. The head of this bit is 1/4" and it fits down into a 1/4" hole and can then be used to remove core, for potting, without first over-drilling the top skin to 5/16".
I should mention that I personally prefer to leave as much of the top and bottom skin in place as is possible. Much of the strength of the deck comes from these skins so leaving them intact when potting hole is what I consider a good practice. Leaving the top and bottom skins intact, without over drilling all the way through both skins, leaves you with a "captured plug" of epoxy that can't physically pop out or break free as easily as a complete over drill of both skins can. A number of years ago I set up some destructive testing in my own shop to see which was stronger. While both are strong I found the captured plug to be stronger.
Either method is arguably far better than what the builder did, and that is not sealing the core at all. Don't fret over the method just chose one and do it before it's too late.