A couple of thoughts......
You're in relatively the same area I am, so I'm going to guess that the boat has sat wrapped up all winter. A lot depends on how it was stored (e.g. shrink wrap, plastic tarp, canvas tarp), and more importantly, how much air was able to move from inside and outside the covering as well as (and most importantly) into and outside of the boat. The less air movement from inside the cabin to outside and from inside the wrap to outside, the more condensation you're going to get. Proper ventilation and air movement are the bigest keys.
I prefer shrink wrap because it allows for a vents to be placed anywhere and everywhere. The more the better. I also make sure my shrink wrap goes belong my portholes. I then open the port holes and crack the hatches to allow for air movement both inside the cabin and from inside the shrink wrap.
The rest of the moisture is taken care of by copious amounts of Damp Rid. The BIG containers and several of them. These need to be swapped out monthly.
I remove the mattresses from the boat completely. In fact, EVERYTHING comes off of the boat. If I absolutely can't remove the mattresses, I'll stand them up on end in the companion way if I have too. Leaving them laying in place will create condensation and possibly mold.
To clean, you should be able to remove the covering. There should be a zipper along the edge. The foam mattress can be air dried or you can try and clean it if you want. The covering can be washed like a gentle item. Make sure both are bone dry before putting the mattress back into the covering.
Best of luck.