Bought a swim platform for my Topgun. I am going to thru bolt it. Some will be below waterline and dont want any water intrusion.
Do I use resin to seal holes?
Do I use 5200?
Do I use both?

I'm sure the experts will chime in but I would oversize all the holes and fill with epoxy and then redrill to the right size ...then 5200 ....m

I second the oversize hole filled with epoxy, then drill correct hole. This keeps water out of the core.

How much bigger?
Anybody done this? How hard?
Thanks

The above would certainly work but it's overkill. If you caulk it correctly inside, outside and on the bolt with 5200 you shouldn't have a problem.

Check your placement several times, and check the inside before you drill. It might seem obvious but plenty of people get hurrying and drill right into a wire harness, or a hose, or dead into the middle of a stringer.