Boat mattress Sizes are quite impossible to pin down. There are NO generic sizes. Often a boat mattress may also be used as a sitting cushion so many boat mattresses are between four inches and six inches thick.
Boat mattress beds are almost always custom made. Boat manufacturers try to use every nook and cranny of a boat which means that the boat mattress often winds up being cut with curves and odd shapes.
Many people these days purchase a memory foam boat mattress product because it lends itself to be cut into odd shapes. This is almost impossible with an innerspring mattress. Memory foam works through your temperature to mold to your body and eliminate pressure points for a better nights rest. You may want to consider a memory foam overlay over your existing boat mattress to save a ton of money and add comfort.