The ~15 minute lifetime comes from needing to continuously tread air to stay aloft. (You can use up the battery faster by going fast, but there is a max time to just hover.) In contrast, a water-density vehicle could float around and leisurely collect data for almost unlimited amounts of time so long as it doesn't need to get anywhere quickly. (Currents will complicate this.)

15 minutes is a long time. My Trex 450 in idle up mode has about 4 to 5 minutes max of flight time.