Sorry, I was away for a while. This is in reply to the previous 3 postings by sfabtx.
Spotted Dolphins can be seen pretty much yearround off of Bimini and other areas in the Bahamas. It might take a couple of hours motoring northwards to get to the dolphin grounds. Spotted Dolphins are quite sociable and friendly - even more so than Bottlenose - and over the years, they have apparently become accustomed to snorkelers due to a number of boats that specialize in dolphin encounters. Like all dolphins, they are very playful but get bored easily if snorkelers just float on the surface. Clown around doing antics, and freedive and they stay around. Sometimes they can be seen with Bottlenose Dolphins as well.
The dive operation I went with, Neil Watson, offers Spotted Dolphin snorkeling trips - I am not sure what the minimum number of snorkelers is, but I know that a group of about 6 divers that were there doing the Great Hammerhead diving when I was there, arranged with Neil Watson to go look for the Spotted Dolphins one afternoon and were able to see and swim with them. If you go to the Neil Watson website, you should be able to find info on this.
As an aside, if swimming and freediving with these dolphins is something that really interests you, there is a liveaboard operating out of West Palm Beach called the Dolphin Dream that does 7day/6 night trips to the Bahamas and sometimes to Bimini in the late spring and summer that are pretty awesome, specially when Scott, the captain, takes you out to snorkel with them at night.
As best as I know, Bimini is the only place right now where you can get consistent sightings of GHs.....and up close. That is not to say that you can't see them elsewhere in the Bahamas. I know that they sometimes see them at Tiger Beach, and I was told that there is one large female that was once known to roam around Bimini but was last seen at Tiger Beach.
Lastly, I don't know how many different dive operations there are in Bimini, but I can tell you for sure that Neil Watson's operation is located at the Bimini Big Game Club, and it is the only one there. I believe that Bimini Undersea is the dive operation located at Resort World, which is waaay north on North Bimini Island. During the 5 days I was diving in Bimini, we didn't see any Bimini Undersea boats in the Hammerhead dive area, just the boats from the Bimini Shark Lab, Epic Diving, and two liveaboards.