You need help from the other guys in FL.
I think it will help some to provide air circulation, so leave the cushions
tipped up when you leave the boat.
Also, you could try the clove oil
Basically, your boat is in a very high humidity area, which is why so many people run de-humidifiers.
Good luck on trying to find a way to cope with it. As far as I know, the extremely high humidity is a very difficult environment
in which to keep things nice!
on edit: You might report my post to the Mods, [click on the triangle in the lower left of the post box] and see if they can add your location to the title of this thread, and fix the spelling of "Upholstery", it might make answering more attractive to those guys in your area who are combating the same problem.