Underwater hockey is a great sport (yes a real sport) played on the bottom of a (usually tiled) swimming pool.
It is played all over the world and currently Australia is world Champions (as of world championships in year 2000).
Players use standard snorkel gear (mask, snorkel, flippers) and a stick about 8 inches in length that is used horizontally on the bottom of the pool. A plastic coated lead puck is pushed and flicked around, the aim being to get the puck in the opposing teams goal.
Teams consist of 6 players in water and up to 4 subs.
People who play develop high cardiovascular fitness levels and team skills like any other team sport. Its great cross training for any water sport, in fact it was apparently designed by British divers to increase fitness.