I travel a lot with work, invariably finding hotel mattresses to be almost as comfortable as stone, so have adopted an old camping trick. I always bring along a Thermarest pad (tried other cheaper brands, but they are just not as comfortable), throw it atop the bottom bedsheet, and inflate it to whatever level feels best that night. Since using the Thermarest, every night is comfortable, no matter the bed.
This approach should also work on a boat. Even if you don't want it as a permanent solution, it would make sleeping comfortable until you can get cushions more to your liking. (At which point you can relegate the Thermarest to use as a spare, easily-stowed, extra mattress.)
Regards,
Leander
leander@worldnet.att.net