The Full Bathroom Cabinet
Crewed yachts stock hotel-grade shampoo, conditioner, and shower gel, and there's a proper hair dryer aboard - ask the crew before you pack your own, because plugging a personal high-wattage dryer into yacht power isn't always straightforward anyway. The mega-sized toiletry kit is dead weight.
Two warnings in the opposite direction. First, the things you should bring: reef-safe sunscreen (several Caribbean jurisdictions restrict oxybenzone formulas, and the crew will thank you - see below), your prescription medications, and a motion-sickness remedy even if you've never needed one. Second, the thing to leave out of that cabinet: aerosol tanning oils and sprays. They stain cushions, turn teak into a skating rink, and many crews quietly ban them.
What veterans do instead: sunscreen, meds, a toothbrush. The rest is already aboard.