 You could use a porch and floor paint, but really, the solid color stain that's used on decks, you're going to at least double the lifespan of a semi-transparent. Now, there's sometimes on decks, people don't want that painted look. They want that, I want it to look like a wood floor look, and that's fine. But on a dock, maybe cosmetic's not as important. That protection is more important, and that's why I'd recommend that.