I have two uses:
1) the covered porch (roof, but no sides) t&g floor when I'm done with that project
2) a 24' x 8' pontoon with marine ply floor to be used as a dock in my pond.
On #2 I have applied 2 coats of Valspar Oil (urethane) porch paint. That ply is like a sponge. It could easily drink up another coat, but the ones on it are a bit rubbery still, so I plan to leave it be for now.
Would a good water base be just as effective? I don't mind -- in fact prefer -- the idea of some annual touch-up instead of dealing with oil cleanup.