the pads can help in some cases, but not all cases.
the mattress pad protector/ventilator is usually desired on boats that sweat and the moisture runs down the inside of the hull to soak into the mattress..... using one is an attempt at solving a symptom and saving the mattress from the real problem.
a wet bilge or leak is almost always the cause of a chronic problem, because the water is already in the boat, never leaves, but continually gets recycled as it evaporates and then condenses again.... the ventilation pads very rarely help in this case.
dry the bilge and keep it dry, and then add full time ventilation to the boat somehow.... this solves the problem in almost every case.
however, in warm humid environments, depending on the mattress thickness or material, when people are sleeping on it and if it lets moisture thru to the mattress deck, it may condensate, and a ventilation pad will help in this case
some boats have a carpet type stuff on the walls, which on a wet boat, can become horrendous with mildew/mold and saturation, but on a dry boat, I repeat on a dry boat, the covering offers benefits.
it offers a bit of insulation for sound and temperature, and, when there are people aboard sleeping/cooking/partying/living, the covering helps to prevent the natural condensation from forming on the hull interior and running down to the mattress and bedcovers.... what little condensate that may form is wicked away from the hull in the fibers and will airdry with the air movement within the boat due to the people moving about or a fan running..... but this evaporated moisture still needs to be ventilated from the boat, or it will accumulate and cause dampness in everything....
for those who want to have a ventilation pad under their mattress, the cheapest, yet fully functional is a product called foundation drain board. it can be found in different styles from different manufactures, and a piece big enough for a bed can usually be gotten for free from a contractor who pours concrete basement walls (its used as a ventilation pad on the outside of buried concrete walls to allow water to drain away and not stand against, and leak thru, the wall), and IIRC, I read somewhere a few years ago that the first type of this product was the where the ventilated mattress pad used on boats was born from.