Boat carpet is made of solution dyed polypropolene similar to indoor/outdoor or kitchen carpet. It is very stain resistant and will stand up to some pretty vigorious cleaning. I clean up coffee stains and the like with automotive foam carpet cleaner. It works great--just foam it on and scrub it with the brush and wipe off with a damp towel. The real test will be cleaning up some chartruse Spike-it my son spilled last weekend.
I was in the carpet biz for 10 years--we would demonstrate the color fastness of kitchen carpet by pouring full strength bleach on it and rinseing it off. KIDS DON'T TRY THAT AT HOME.
Harpo