We learned that ventilation is one of the most important things to summertime comfort on the boat. Wish I'd known about these neat fans when we were upgrading ours -- they're a lot better than the solar vents we used!
V berth mattress - We read in sailboat forums about the Spa Sensations memory foam mattress from Wal-Mart. We got the 8" Spa Sensatins Memory Foam Mattress for $260 (now $199). We measured the dimensions of the v-berth, then drew it on the mattress. He cut the mattress with an electric bread knife which was easy and didn't tear it.