I have an Airaid with an Outerwears pre filter. I used to use them on my Seadoos and they worked great. They are treated with a water repellent, similar to the stuff used on tents etc, that will prevent drops/splashes/spray/etc from penetrating the filter. If it was totally submerged the vacuum from the intake would no doubt pull water through it.
The water repellent treatment does not last forever though, and they need replacing every few years. They say to replace them every year I think, but I've found that they always last longer than that. You can easily test to see if it is still working by dropping a couple of drops of water on it, if it beads then it is working, if it soaks into the mesh then it needs replacing.
The mesh is very fine and keeps debris/dust/dirt/etc off the filter. The mesh is not so fine that it causes any noticeable restriction on air flow, but will help keep the filter clean, thus better flowing.
For what they cost it is well worth adding one in my view.