use marine fabric if you can.
We had our cabin cushions recovered 2 years ago,,,we had them done in dark blue with white piping,,,they looked beautiful,,,until 2 things happened.....the first summer we had them,,,our son came down from swimming...he was dry but his swim suit was not....he sat down...when he got up his white swimsuit was blue,,,and we had a nice wet circle on the cushion.....second happened when we wernt on the boat.....our deck hatch sprung a drip leak on a driving rain,,,,that left another water mark on the cushions....the wet blue then bleed onto the white piping..... we flipped the cushion....I tried scotch gaurding but that will be a patch....try anything that is water resistant and stain removable...they will take a beating.