How can shiplap be sealed to withstand moisture in a bathroom?

I started going through these videos, there's so much good info in all of them I'll just send you a whole page! Have fun with it!

https://www.youtube.com/results?search_query=shiplap+bathroom

Marine grade epoxy or poly

I would go with the marine polyurethane. I used it when we replaced the back steps that were rotting really quickly. We replaced it almost three years ago and it has withstood all the weather that Minnesota gets and the four dogs constantly going in and out the dog door.

We did not seal ours - just primed and painted.

Unless the wood is lining the inside of the shower I don't see the need for anything more than a good prier and semigloss paint. You want it to withstand moisture not become waterproof, right?

That should be a good "primer" and semigloss paint.