From my experience i would not recommend using Sunbrella for seating fabric. It gets very grubby very quickly. If you are concerned about the cushions getting wet, I would suggest covering the cushions with some sort of completely waterproof material like vinyl and then fitting loose covers of some sort of genuine upholstery fabric, preferably one treated with Scotch Guard. If you can't afford that, at least you will have covers that are easy to remove and put through a washing machine.
Fantail has her original plush covers, professionally made, which don't seem to show the dirt at all. On my previous boats, I've always had problems keeping the seat covers looking more or less respectable. I've used upholstery fabrics, but its never been protected with Scotch Guard. Cockpit cushions covered in Sunbrella were impossible to keep clean.