lol; I think they mean the boat / pump thing will be underwater, not the jetpack. Divers are limited by physiological things when ascending and descending, not how fast they can swim. SCUBA certification organizations will tell you one foot per second up and down is about the limit. They already have underwater propulsion things (little units you hang on to that you point in the direction you want to go, and they run of batteries) to combat currents, long distance requirements, etc. that are about as good as required.