What the sensors do over water
A DJI drone holds its position with GNSS and, close to the ground, with its downward vision system as well. The Mini 4 Pro manual says the positioning function of downward vision is meant for when GNSS is unavailable or weak, and that it works best from 0.5 to 30 m. Then it lists the surfaces where vision cannot work properly. Water and transparent surfaces are on the list, along with highly reflective surfaces, moving surfaces, monochrome surfaces and areas where the light changes often. A lake on a sunny, windy day can tick four of those at once.
The manual spells out the consequence: downward vision may not work properly near water, so the drone may not be able to avoid water below it when landing. DJI's advice is to keep control at all times and not over-rely on downward vision. The same goes for the pilot assistance system (APAS), which the manual says may not work properly over water or snow, and for FocusTrack, which should not be used near water or glass.
The Mini 4 Pro manual also gives a minimum: among the flight environment requirements, it lists avoiding bodies of water, with a recommended height of at least 3 m above water. An older DJI guide on flying over water, republished on the DJI Store Sofia site, gives 2 m and suggests turning vision positioning off if you have to fly lower. That guide was written for the DJI GO 4 app. Treat the number in your own manual as the one that applies.
Turning vision positioning off
On the Mini 4 Pro, vision positioning and obstacle sensing can be switched off in DJI Fly under System Settings > Safety > Advanced Safety Settings. Read what that means before you do it. With both off, the manual says the drone hovers on GNSS alone, omnidirectional obstacle sensing is gone, and the drone no longer slows down automatically when it descends close to the ground. The setting turns back on after every restart. In RTH, auto landing and Intelligent Flight modes, vision positioning and obstacle sensing are not available anyway, whatever the setting.
The worst case is when GNSS weakens too. With vision unavailable or disabled and a weak GNSS signal or compass interference, the drone switches to ATTI mode. The manual warns that in ATTI the drone can't hover or brake by itself and drifts with the wind, and says to land as soon as possible. Over water, "land as soon as possible" means flying back to the shore without position hold, so practise flying in ATTI over land first.
The home point and failsafe
From the shore, the usual rules apply. Take off from land, wait for the "Home Point updated" voice prompt and set an RTH altitude that clears everything between the drone and you. The Mini 4 Pro manual gives a default RTH altitude of 100 m. It also warns that RTH may not avoid transparent objects such as water and that you should take over manually in an emergency.
Go through the failsafe action too. In Advanced Safety Settings, the action when the remote controller signal is lost can be RTH, Descend or Hover. Over water, Descend means landing in the water. RTH to a home point on the shore is normally what you want, as long as the home point really is on the shore.
If you walk along the bank
The home point is recorded at takeoff. If you walk a few hundred metres along a river while filming, RTH will fly to the spot where you started, not to where you are now. DJI Fly lets you update the home point in flight under Settings > Safety > Update Home Point. A DJI support reply on the DJI forum lists three ways to do it: tap OK to set it to the drone's current position, tap the remote controller icon to set it to the position of the phone or controller (which needs a strong GPS signal on that device), or drag the home point on the map.
From a boat
The Mini 4 Pro manual is blunt: do not take off from moving objects such as cars, ships and airplanes. It also says not to launch the drone from your palm or while holding it, and to keep at least 10 m from buildings and large metal structures because they affect the compass and GNSS. A boat breaks most of those rules at once. If you still fly from one, the older DJI guide recommends taking off while the boat is stationary rather than moving. It also suggests a higher RTH altitude near boats and warns about large structures such as container ships and lighthouses that can interfere with the compass or GPS.
The bigger problem is the home point. The same DJI support reply says that when the pilot is moving, for example following a vehicle or a ship, the home point must be updated manually. A boat that has drifted 50 m means a drone that lands 50 m from the boat, in the water. If you update the home point to the controller, keep doing it during the flight. Even then, the manual's RTH behaviour ends with the drone landing at the home point, not hovering for you to grab it, so be ready to take over before it touches down.
Battery margin
Battery warnings matter more over water. The Mini 4 Pro calculates whether there is enough power to return home and starts RTH after a countdown if you don't respond. When the battery can only just cover a descent from the current height, the drone lands on its own. The manual says this auto landing can't be cancelled, but you can still steer the drone horizontally and control the descent speed. Over water that is the moment to fly towards the shore with everything you have left. Cancelling RTH after a low battery warning, the manual warns, can leave too little power to land safely.
Wind adds to this. The older DJI guide notes that strong wind drains the battery faster and is often stronger higher up than on the ground. Check which way it blows before you head out: if the way back to the shore is into the wind, the return leg will cost more than the way out. The Mini 4 Pro manual says not to fly in winds above 10.7 m/s, and the closer you are to that, the bigger the margin should be.
FPV drones over water
An FPV drone has no vision sensors to fool, but it has its own weak points.
- GPS Rescue, stage 1. Betaflight's GPS Rescue guide says the stage 1 failsafe throttle should make the drone climb steadily. If it is set too low over water, the drone can descend enough to end up in the water in the one second before GPS Rescue starts.
- ESC protections. In his AM32 guide, Oscar Liang says he disables the temperature and current limits on his FPV drones to avoid sudden power loss mid-flight, and points out that a sudden loss of power is a particular problem over water. For low voltage cutoff in the ESC, he says it is generally not recommended on multirotors at all.
- Height. Flying fast and low over the surface looks great on video, but there is no room to recover from a stutter, a failsafe or a gust. Spray is another reason to keep some height. Even the Mini 4 Pro manual lists salt spray among the conditions in which the drone, controller and batteries should not be used.
If it goes in
DJI's drones are not waterproof. The Mini 4 Pro FAQ says so directly. The manual warns that turning on an aircraft that has fallen in water may cause permanent damage, so don't switch it on to check whether it still works.
The battery is the dangerous part. DJI's Intelligent Flight Battery safety guidelines (in this case for the Mavic Pro) say that water inside a battery can cause chemical decomposition and may lead to a fire or even an explosion. If the battery falls into water with the drone, take it out immediately, put it in a safe, open area and keep away from it until it is completely dry. Never use it again, and dispose of it properly. Don't heat it to dry it out.
Before you fly
- Read your drone's manual on water: minimum height and what vision positioning does.
- Take off from land, wait for the home point and set the RTH altitude.
- Set the signal lost action to RTH (not Descend) with the home point on the shore.
- Walking along the bank? Update the home point.
- Keep a bigger battery margin than over land and plan the return into the wind.
- FPV: check the GPS Rescue stage 1 throttle and the ESC protections.