There is another point here.
Marine engines usually have flame arresters on the induction system. There's usually NO filter because it's not needed.
There's very little (if any) dust or other contaminates on a lake or other water way. The air is clean .......so, you don't really need a "filter" per se,
The flame arrester will stop leaves and other large "things" from being sucked into the engine. It also will usually flow far more air than ANY filter (including all of K&N's filters)
There's also the small thing of USCG certification. Any time you get stopped by the "Boat Cops" the second thing they check (after the fire extinguisher) IS the USCG approved flame arrester......mainly because it's right up there on top!
(and of course......you probably should have an effective flame arrester on the engine too!)
Cheers,
Rick