A lot of the mold under the mattress issues are less to do with humidity and more to do with condensation, it's why it's much more a problem in smaller boats, as the space under the bed is typically a locker against the hull and always cold.
The two ways to fix it are to improve the movement of air, or stop the condensation all together.
I've used the 1/2" mesh to keep the mattress up off the surface, and it mostly just gave the water a space to condense without soaking the foam, but it still formed constant puddles that had to be mopped up.
Insulating the UNDERSIDE of the lockers with enough thickness to keep the dew point within the foam meant no condensation on either side of the plywood, and the mesh still fit under it to allow it to breathe.
When things were constantly damp during a particularly miserable winter, before insulating the bed(it was too wet to stick anything on) I used the very thin plastic painter's drop cloth plastic, wrapped the foam in it, and "stitched" the edges together using short strips of tape every couple inches, I faced the stitched edge towards the center of the cabin, not the outside edge. This allowed the mattress to breathe and move normally, while protecting it from water underneath, and spills on top. The foam I did this to stayed mold and moisture free. The bed also felt drier all the time, and the thin plastic was not noticeable at all. After a year it was still working perfectly.
I saw a great possibility in combining the insulation with the ingenious map and large flat document storage compartment someone came up with here, but I can't find the link for that. That air gap would reduce the insulation requirement a lot.