The perfect solution would be CCushions. Will not absorb water at all.
Might cost more than foam covered in Sunbrella but worth it. You just send them a paper template and they make it from that.
Click on link below
I know you are looking specifically for fitted cushions. But if you want something cheap & easy in the mean time, I found some long, skinny bench cushions for outdoor furniture at Garden Ridge/@Home a while back that were pretty close in fit and kept passengers comfortable on leisurely cruises.