Lots of variables but generally speaking use open cell (so it breaths) polyurethane foam, not foam rubber. Use "firm" (around 50-60) foam, especially if the seats are 2" thick or less. Fabric can be standard marine grade acrylic (Sunbrella) or standard vinyl sold at any fabric shop. Marine grades are generally better fitting (stretchy), longer lasting and UV resistant but I've used several different retail store vinyls without problems on live aboard boat interiors. Stretchy type vinyl fits better, stay tighter and are more comfortable after long term use so that's what I use. Name brand fabrics really are better but you pay the price. Good luck. Also, if you are doing exterior seats provide drain holes in the bottoms.
