You can just used regular sheets
, sized large enough for the bottom cushion(s) on your primary bunk(s). And the only "trick" is to buy a few sets of sheet stays, so that you can "tune" your sheets to fit snugly.
Ah, that, & to make sure that you have a good moisture management system... for the bunks & cushions
, as well as the boat, given condensation
levels underneath of bunks when the weather's cool to cold.
PS: I forgot to add that you may want to try out something with a bit of a synthetic blend(ed with cotton). As salt's damn near impossible to get out of things, especially if you live aboard. And once it's in sheets, given it's nature (hydrophilic), they'll almost always feel a tiny bit damp & cool, on all but the hottest, driest days... even in spots like SoCal. - Just a "think".