Even if the vehicle is submerged and the water goes down. Its likely water has made its way into the engine. The first thing to do is open your engine air filter and check for moisture.
If it’s moist or damp, DO NOT CRANK THAT VEHICLE!!! DO NOT ATTEMPT TO START IT. .
Allow the water to go down, tow it away if possible. Get your mechanic to drain the engine oil immediately. Also drain the transmission fluid. Do not take the risk of trying to drive that vehicle.
Let the mechanic disconnect the fuel sending unit (fuel pump), pull out the spark plugs one after the other and then crank the engine. You want to make sure water in each cylinder is blown out of the engine by the compression / exhaust stroke.
Once completed for all the cylinders. Connect the fuel pump (easiest way is to remove the relay or the fuse) and start the vehicle. It may idle rough. Leave it. Do not rev it. Allow that vehicle idle for some 30 minutes
Once it’s up to operating temperature, (because you have replaced your transmission fluid) engage the gears one after the other. From P to R wait 5 seconds R to N wait another 5 seconds N to D wait again. D to N again N to R and R to Park. After this. You can drive.
Personally once I get the vehicle going again, I’d replace both fluids after another week or two depending on how milky they look. Be safe everyone. Happy motoring.