If something like a kayak is mostly stored and just occasionally brought out for use, I wouldn't bother worrying about protection. I also don't like the idea of slathering on chemical and then dropping it into a water ecosystem. I'm not an enviro-nut or anything, I just try to be thoughtful and avoid things like that when I can. But I'd certainly consider applying some 303 after use if storage options were limited and the kayak was just going to be lying in the sun for extended periods. I'd even wipe it down with a damp towel once a month and reapply the 303. After sitting outside for awhile, I'm sure I'd want to wipe it down again with a damp towel before use anyway, to get all the dust off.
I actually did an experiment last fall. Two identical lawn plastic lawn chairs that sat out all winter. One with 303 applied and one without. The seats of the chairs hold water. So, after rain/snow, there was always a little puddle until it all evaporated. I just recently went out and wiped both chairs down with a wet rag. They were dust covered and dirty. The one with 303 applied last fall wiped completely clean very easily. The non-treated chair has evaporation rings that required some heavy handed scrubbing to remove. And it still left some traces of mineral deposits. Even though people will say 303 falls off in weather (and it does, to a good extent), I'm convinced it does what it's supposed to do and does it well, even if you can't "see" the protection throughout its life cycle.