You want marine vinyl. It is sold by the yard at most marine shops (not stores) and the automotive upohlstery shops can get it for you. Don't know any specific locations in Lakeland but I am sure there are a few.
When it comes to the cushion, you want closed cell foam also. It is a little more money, but it will not absorb the water like normal foam (cushion) will. It is also a little harder on the butt as well.