Not that it answers "how low above water is safe" but would not one way of testing the range of the sensors etc. be to take off on a significant slope and go high, well over the 30m. Then, staying well above 30m AGL, fly either up slope or down slope until lthe ground directly beneath the drone is either significantly higher or significantly lower than the home point. Then descend vertically at maximum speed and see where, AG, the drone starts to automatically slow its descent. I presume the drone does not know the height of the ground relative to the take off point so it must be measuring or be attempting to measure it's height AGL during the descent if not the entire flight .
I can't test this myself here, sorry.