Probably this was already discussed on the past.

Should I turn off the down sensor while flying low (1m) over water?

Hello and good day Adriano Araujo. Thank you for reaching DJI Forum. It is not recommended to fly the aircraft close to the water. When flying above the water, the vision positioning function will turn off automatically and only the GPS and barometer are used for positioning, if affected by the accuracy range, and an accident might happen. Thank you.

So I don't need to turn it off, as you said, it will turn off by itself?

Adriano Araujo Posted at 2-25 10:05
So I don't need to turn it off, as you said, it will turn off by itself?

Hi there, thank you for reaching out. With regards to that, It will depend on how far or near you will going to fly above the water. However just to heads up if ever it's near that is the time the sensor will confuse about the reading and it will notify you as well about if it's an obstacle or not. Fly safely. Thanks!

If I fly over water and I'm going to be low over it I will turn mine off just to be on the safe side

You should definitely turn off the sensors. They might get confused, especially that low over water which might result in an auto land on the water if you don't react fast enough.

darrell2 Posted at 2-26 00:13
If I fly over water and I'm going to be low over it I will turn mine off just to be on the safe side

Without the Botton sensor (just using the barometer and GPS) dois it maintain a good altitude and attitude?

Adriano Araujo Posted at 2-25 10:05
So I don't need to turn it off, as you said, it will turn off by itself?

I don't think this is right, the problem is the VPS can't accurately tell the distance when over water, snow, ice anything super reflective etc at which point how is it going to know it should switch off?

With it on you run the risk of the craft wanting to land, choice is yours.

Also given the VPS only works to about 15m and the crafts maintains a very good altitude when it's enabled but not in use that should answer your other question re baro + gps

i would not recommend flying that low over wather, but if you do, make sure to disable the sensors and good luck!

It makes no difference if the sensors detect the water surface or not, your concern should be if the water snages the aircraft. It will then be dragged under.
Landing is determined when the craft is expecting to decend but is no longer moving down. Then it stops the props. The under side sensor only detects the ground as an obstacle to pause decent (to reduce speed and search for a suitable surface) and if stick is still down,  to continue controlled decent in expectation of landing. Sometimes that surface is detected as problematic and a manual landing has to be made. I don't believe you can land if the aircraft is moving horizontally, usually forward.
If you want to see instances of very low flight over water, see this thread.
https://forum.dji.com/thread-182111-1-1.html
Especially #13   Lucky for some.