We bring masks and snorkel and fins. Wife and I cram our two sets of each into an over the shoulder dive back that is about the size of a set of fins, slightly smaller (narrower, but longer) than a standard backpack, and still leaves room to spare for some personal items. The other carries a backpack and we each load up our rolly bag. Carry on only, one bag in the overhead bin and a personal item under the seat and we've got everything we need for a week or ten days in the BVI. Phone charger, ipad, a book or two, a pair of jeans, light jacket and some swim trunks and t-shirts and the toiletries.

We spend a lot of time snorkeling, so to us it's totally worth it to have a well-fitting mask, snorkel and fins. If we didn't bring them, not sure what we would use the extra room for, although I make have to bring my laptop for work this trip, but I'm pretty sure I can still get that into my rolly bag.