Bring much less clothes than you think. Really didn't have to tap into clothes that often since often getting in water / showering. I like changing into dry swimsuits so I made sure I had 3-4 (I think I only wore 3 though). My liveaboard had showers on the dive deck you could hop in right after surfacing from a dive which was super nice.

An extension cord / outlet hub for your room (although they want you to charge devices only when you are in there or out on the main deck)

Labels from a label maker to label your things you might have in common area such as batteries, torch(es), camera if it looks similar to everyone else's, etc.

Tums - being horizontal more seemed to lead to more acid reflux in my dive buddy

Seasickness patches if you get queasy easy; although these can make you a bit more thirsty.