If not rare I've found it to be uncommon to encounter dolphins on any one trip. I'm sure over the years that many came alongside or surfaced at the bow that we might have missed. People are busy chatting etc and often not looking close to the boat when out of harbors. Often their "swim alongs'" are brief and easily missed.
Dolphins are somewhat rare in the BVI because that gin clear water we all love is not conducive to the food chain that supports them in other waters.
The most amazing dolphin experience we have had was in the backwaters of Charleston South Carolina. There is a guy who gives trips there out to Sullivan island, near fort Sumpter. His family has had land nearby for 200 years and has a lot of civil war and post civil war information. But the highlight of the cruise was when he picked us up from Sullivan's island and said Dolphin were feeding off the other end? We had seen a few dolphin on our way out to the island and I wasn't as enthusiastic as I would have been if I knew what he was talking about. We went down a small strip of water between marshy areas and oyster beds on each side. You saw several dolphin close by. They were herding mullet together. Then with a rush 7 adult Dolphins rushed the school up onto the shore coming totally out of the water themselves on a dirt bank between the sharp oyster beds. They grabbed mullet on shore on the way to the shore and as they went back into the water. Each dolphin turns on its right side and works it's way back to the water. They then herd em up again, repeat the process of beaching themselves to feed until full or bored and move on. I thought watching whales work together to bubblenet feed in Alaska was the greatest, but this beats it. One of the most amazing things I've seen..and you were 40 feet away as it was going on. It is called "strand feeding" and only happens with a line of Dolphins in South Carolina and Georgia. We took videos but if you google strand feeding Dolphins you will likely get a better picture.