No don't say wet suit material as wet suit material is total different, sorry Daniel.
just simply ask for GRIPPER seat material for motor bikes or jet skis.
You will get a better price if you go to a auto trimmer. I got a meter GRIPPER for a jet ski as it is water proof for $33. It was a 1 m x 1.7 m, enough for 4-5 seats.
It is cheaper for the non water proof stuff but it is a bit stiffer and a bit harder to stretch and fit over your foam seat.
If you used neoprene as a seat covering it wouldn't wear to well, but would be nice on the butt though.
Also if you got it wet and didn't leave the thing to dry in the sun for day, every time you got on it you would get a wet arse as it holds water very well.