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You can buy cockpit cushions made of 2" closed-cell foam covered with Sunsure, a woven fabric made with PVC-coated yarn from this site (link below). The cushions float, dry quickly, won't absorb water, and are resistant to mildew, rot, and UV rays. They also snap securely into place on the benches, so they won't slide around when the boat heels with snap hardware provided. I installed these on all my sailboats.
Cockpit Cushions, Sunsure Fabric
shop.sailboatowners.com