Hypervent worked great for us. After 1 season we were starting to get mildew under the new mattress. After 7 years the hypervent is working great. Dri-deck tiles would also work but may be even more expensive. Anything to get air circulating under the mattress.

Moisture will accumulate on any cold surface. If you use an air permeable insulation such as polyester batting in your locker the water will collect on the fiberglass. You can try to use a closed-cell foam and seal the edges but unless it is absolutely air-tight moisture will still find it's way in. I would use blue-board Styrofoam cut to fit and scored as necessary to fit any hull curve. Moisture will still collect behind but will drain down into the bilge.

We don't live aboard but do run a demumidifier in our Pacific Northwest winter that keeps everything nice a dry. When we spend a few days aboard it collects a huge amount of water. The condensate drains into our shower sump which pumps overboard above the waterline.