I wouldn't use galvanized bolts. Although in theory they should hold up fairly well, today's box store bolts are not the same quality that they used to be. The electroplated hardware is crap . Secondly, some marine cleaning chemicals will harm galvanized metals. So get good marine grade SS hardware including the washers. For smaller bolts 4200 is OK but for larger ones and the holes, I don't recommend 4200 or 5200. It may make it very difficult to remove the hardware if needed in the future.
Note that in addition to bedding the bolt heads, the hole in the top should be sealed. For bigger holes, I would use epoxy. For smaller, caulk like Life seal will do fine. Just make the hole two sizes larger in diameter than the bolt so that inserting the bolt does not wipe the sides of the hole clean.
One point about nylock nuts. I use them for applications like yours because that can be tightened lightly but won't loosen. Regulat nuts and lock washers have to be tightened fairly tightly to stay tight and the heavier tightening force may compress or deform the cored top.
Note that in addition to bedding the bolt heads, the hole in the top should be sealed. For bigger holes, I would use epoxy. For smaller, caulk like Life seal will do fine. Just make the hole two sizes larger in diameter than the bolt so that inserting the bolt does not wipe the sides of the hole clean.
One point about nylock nuts. I use them for applications like yours because that can be tightened lightly but won't loosen. Regulat nuts and lock washers have to be tightened fairly tightly to stay tight and the heavier tightening force may compress or deform the cored top.
