Bring a large (no skid bottom) cup for water/ juice/ beer/ rum punch they provide a small cup that you hang on a hook when not in use but you'll appreciate having your own.
As far as the clips ensure they are HEAVY DUTY for hanging your wetsuit on the lines to dry.
Don't forget your Bonine/Dramamine just in case.
You'll also appreciate a sports seat cushion those benches get hard after awhile.
The dive gear storage area is under the benches and will be shared with your dive buddy they are fairly roomy infact you get more room for your gear then for your personal stuff. Speaking of personal stuff DO NOT bring alot of clothes items etc. you get a 3'x1' shelf to store ALL of your personal items.
Luggage is left on the dock in a connex during the trip so I wouldn't worry to much about how bulky it was.
Ear Plugs for sleeping may be advisable especially if your in the main galley. The 6am wakeup from the cook banging around or the late night card playing from the other passengers may keep you up.
Most important bring a good attitude and you'll have a great time.
it really is camping at sea!
I went in Dec on Sea Explorer and loved it.