I carry on much as I can get away with to make sure I'll have that much in case checked bags run into problems. I remember a US jet in Roatan last year with luggage doors that wouldn't open and they almost flew it back that way for repair which would have meant no one on the plane would get any checked bags for at least a day. Fortunately they just barely did before giving up, but checked bags sometimes arrive days later. Any clothes I want to change into for 24 hours are in my roll-on along with as much gear as I can: regs, computers, masks, strobe, and anything else that will go. I usually have a couple of 1# soft weights in my roll-on as I like those when the Op has a limited selection of weight choices, and all batteries, to reduce weight in my checked bag.
Roll-ons are often gate checked tho, if you have to fly a leg on a regional jet or other reasons, so I carry all cameras in my backpack with medicines, papers, passport, money, the most important stuff - and the backpack never leaves my body, unless I leave it with a trusted friend while I go to the lavatory. Ok, maybe it goes in overhead if there's no enough room at my feet, but I prefer at feet for that one.
If you don't have a digital, hand held luggage scale, they're cheap on ebay and will prevent airport problems at check-in.
I suppose you have DAN dive insurance
and not the bottom plan as it's a dog and only $10 less than the middle plan that is much better. I like comprehensive trip insurance when I leave the US, but that's a personal call. Make sure you personal medical coverage is good in the USVIs, or consider trip insurance. I use TravelSafe.com as it covers a lot of losses reasonably.
St.John is almost all national park and wonderful to explore by rental car. Hope you can mange a day but it's difficult to get to from St.Croix.
Have a great trip! 
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