  Following with the diver's theme from before, a snorkel jetpack falls into category. It goes on your back like a backpack, and has a battery, scuba tank, emergency air tank, emergency air release tank, and a propellor. It pushes you along with the propellor, also carrying your compressed air. In the case of an emergency when your air runs out, it switches to the minute long air resevoir, and takes the compressed emergency release tank and shoots all of the air out, launching you forward at intence speeds.
This could be a bad idea if the diver is facing downwards when they let out the release resevoir, launching them deeper into the water. While it shoots out the air it could also be filling a shoulder tank that fills to float, so it floats your head up, aiming it upwards, and making you float faster. Also that would keep your head above water once you are at the surface. 
