Your decision, but sounds sorta' foolish. Any foam seat you get wet will hold the moisture, and any foam seat will be more comfortable and durable over time than a net seat.

Something to look into is NikWax brand waterproof products. They make various products for multiple substances/applications. Their waterproofing supplies accompany almost anyone who has/had trekked up Mt. Everest---yeah, the stuff is that good. It is simply a wax polymer that forms a protective seal---water beads up and runs off in rivers.

I use a NikWax leather/synthetic waterproofer for my boots (one designed for Gore-Tex), and they make one specifically for tents and bags that could be used for seats.

The only obstacle is you will probably have to go to a camping/hiking store to find the stuff, but it's well worth the $$. A single bottle will cost you about $8.

S&R