If you use a desiccant, I'd recommend that you use the kind that fit over a coat hanger and hang them in your lockers. You can find them at your local superstore. I think the tubs, used in the cabin are going to be a pain to deal with, and ineffective unless you really button the boat up. You really don't want the watery residue in the bottom container spilling while sailing; it's nasty stuff.
I'm assuming you have the forward fan on intake and the aft two on exhaust so as to set up a air flow pattern. If they are not moving enough air you might want to go to a higher capacity 12 volt fan vent powered off your batteries. I would set it up with perhaps a solar activated relay so that it switches on at sunset and off at sunrise. Your problem occurs because you're not getting enough circulation in the evening hours when the temperature drops to the dew point. If you're down south, you're going to have a certain amount of humidity that is just unavoidable but you should be able to keep your charts from resembling bounty paper towel.
One other thing that occurs to me is that Nicro solar vents do go bad and, in my case, I assumed it was just the battery which I easily replaced. But I found that even with the new battery it would not run for the complete night which is way out of spec. for them. I think you're supposed to get two or three days out of a full charge. Mine did that for awhile, about a summer, before giving up the ghost completely. I can only assume that the motor was in it's death throes, it was a bit noisy, and that it was drawing higher than normal amps as dieing motors will do, and running out of juice over night. In the sun it operated normally. I'd check them out early in the morning before the sun comes up to ensure they've been running all night without a problem. I replaced mine but there is a thread here, somewhere, on re-powering them with a cheap motor from Radio Shack or something.
“Scientists are people who build the Brooklyn Bridge and then buy it.”
Wm. F. Buckley, Jr.