It sounds a little counter-intuitive, but Rendez Vous diving actually makes up a majority of the tourist diving in the BVI. I suppose it is because the BVI is such a big sailing destination; if they dive ops didn't pick people up from sailboats then they wouldn't have any customers. Usually you can arrange dives are pretty short notice over the VHF with local dive ops (just call them up on Channel 16 in the usual way).
As someone said earlier, most of the BVI dive operators tend to have their own patches, and it is a lot easier to Rendez Vous with a particular dive op when you are in the right area. Rough areas of operation of the main dive companies:
- Dive BVI, based in Virgin Gorda and Marina Cay but operate around most of the Eastern part of the BVI. Only dive operator with regular trips to the Chikuzen (Thursdays).
- Sail Caribbean Divers, have several operations in Tortola (East), Cooper Island and Norman Island. Have a pretty big patch stretching from The Dogs all the way down to Norman.
- Blue Water Divers, based on Tortola in Nanny Cay (fairly central) and Soper's Hole (West). They tend to do most of their diving either on the Rhone (Salt Island) or around Norman Island.
- JVD Scuba, based on Jost Van Dyke. Pretty much have a monopoly on the diving around Jost van Dyke and islands in the North West of the BVI.
All dive operators that I am familiar with accept payment by credit card onboard (the old carbon paper system).
The other thing to mention is that all official BVI dive sites have national park mooring balls on them, so if you have a buddy and are reasonably self confident, you really don't need a dive - you can just visit the various sites and dive them on your own schedule.
Hope that helps. Feel free to PM me with any questions about BVI generally (I am not a dive pro, but I have been diving in BVI for a little over 30 years). When are you planning on coming down?