 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, sometimes on decks, people don't want that painted look. They want to look like a wood floor look, and that's fine. But on a dock, maybe cosmetics are not as important. That protection is more important, and that's why I'd recommend that.