Take a look at the coil spring mattresses, they are cooler in the summer and don't fill up with condensation and mold in the cooler months. I prefer the firm style and have a folding model.
Ok, this isn't for everyone, but it worked really well for us
We just ordered 3 new mattresses from Handcraft Matress in Florida.
The quality & comfort is better than the best 5 star hotel we've ever stayed in. It is literally amazing.
Now, it ain't cheap, but worth every cent.
Downside;
The bed was so comfortable, we just couldn't stand our matress at home after spending a weekend on the boat.
So, another 3 grand later, we have something at home as good as our fwd cabin
I must tell you that their quality & service is outstanding