We have to remember that although most of the cushions are “Sunbrella” or simiar, but there are a few which are actually “Upholustry material” which may need a dry clean.
In a boat new to us, we remove the covers, wash them in basen with a mild soap such as Ivory flakes.
Sunbrella advocates you use their preseration and protection kit: $90. There are similar products from: 303, Chemical Guys, Starbright, 3M, Marine one, and a few others. Basically, you want to clean the fabric, then spray it to get the water proff and soil resistance off. We have used Scotch Guard in the past before all of these speciality kits were abailable.