There aren't that many finishes that will dry well in high humidity.

I'd suggest having a look at the Solarez "I can't believe it's not lacquer" range of products. Dries in a few minutes in UV/sunlight (which you will have a lot of). I'd like to try some but it's either rare or stupidly expensive in the UK. Seen it used by a few decent guitar builders on YouTube. You won't need the grain filler, just the finish.