I did one nearby Nassau in the Bahamas, where the ferry launches right across the road from the Atlantis hotel. The fee was around $80 per adult and something less for kids.
The ferry takes you out to a private island about 20 minutes away. The enclosures are such that the dolphins can jump over the tops at high tide to get to open ocean if they so choose. As the guide explained, the fences are to keep sharks out, and the dolphins are quite content being well fed.