I want to replace my fiberglass swim step with an aluminum one. The swim step on there now is through bolted at the top and has a couple stainless brackets that are screwed to the transom with sheet metal screws. I'll use stainless buttonheads to fill the big hole but I want to make sure they are sealed completely. Also not sure what to use to fill the lower ones.

Then want to seal the holes for the new swim step before the bolts go through.

What's the best sealant to use? Marine grade urethane caulking? Epoxy/resin?