If you're a scuba diver and the small fish aren't cutting it, there are some larger sea creatures you can swim with. Humpback whales are notoriously docile and you can get up close and personal in the waters off the Dominican Republic or the South Pacific island of Tonga. The Silver Bank is part of a marine sanctuary in the Dominican Republic and one of the few places where humans are allowed in the water with these huge ocean mammals. Swimming with the whales is limited to what's called a "soft-in-water encounter." This means that you float near the surface of the water and wait for the whales to come to you, which they may or may not do depending on their level of curiosity. You're only allowed to use a mask, snorkel and fins -- no scuba equipment is permitted. This passive approach is at the heart of all "swimming with sea creature" encounters. The idea is that you're merely a spectator and not a participant.
The dolphin is another sea creature on many people's list to swim with. The good news is that dolphins are generally tolerant of humans. Unfortunately, they aren't particularly interested in us either. Dolphins swim really fast and dive deep, and humans just can't keep up. So your fantasy of hanging onto Flipper's fin while it pulls you around the ocean isn't going to happen. You'd need to swim with a trained dolphin in captivity for that, and thanks to SeaWorld, you can do it. Many wildlife experts decry swimming with sea creatures in captivity because it disrupts the natural habitat, but there's a market for it. The "Dolphin Aqua Adventure Program" at SeaWorld allows anyone over 14 years old to swim with dolphins, provided they pay the $225 fee for 20 minutes of water time.