Agree on skipping the TSA locks - like spectrum says, they're often more of a problem than a solution.
Swimmers ear drops (AKA ear beer) are good when doing all that diving. Assorted first aid stuff - cortisone cream, antibiotic cream, benadryl or equivalent in case you wind up itching from something, sudafed or equivalent, bandaids, etc, people will have lots of suggestions on this and doesn't take much space.
It's easy to take more clothes than you need, you just won't use many and likely won't wear shoes on the boat. I take floppers in case I want to slip something on on the boat and not be barefoot all the time, like on some boats the top deck will get really hot in the sun.
If you have a fish ID book for the region and are tempted to take it, you can probably skip that, as most boats seem to have a variety for people to look at on board.