on the level "crashed cargo clearing" this is your first encounter with the scuba gear. there are 3 located on this map and their red, so their easily found.
all you need to do is grab a scuba tank stand next to a wall, throw the scuba tank at the wall then quickly re grab it and hold onto it for as long as you can, it will eventually stop flying into the air and you will land back on the floor popping the achievement.