Aquasub (also known as planasub) is a novelty entertainment you will hear about if you head to Fernando de Noronha. It is basically snorkelling while towed by a boat. To gain estability, you make use of a resin board that allows you to move up and down, left and right. Put your mask and snorkel, wear a life jacket if you wish, and jump into the water.
Although it would appear from the boat that it moves at a really slow speed, once you are in the water you will realize you need strong arms to keep holding the board steadily while moving. If the sea is calm providing good visibility, chances are you will have some beautiful encounters of the underwater kind. If you are very very lucky dolphins might come across your way.
The information we have on the archipelago is at the entry destination Fernando de Noronha.