A lot of good suggestions already so I won't reiterate them. One addition would be to show them what the BVIs look like by using a simple Google Image search. Once they see the beauty of the BVIs it could be hard to resist. Here's what I would show them:
- Marina Cay
- The Baths
- An underwater snorkeling shot
- Cane Garden Bay
- Zoomed out picture of the islands
Good luck!