You got the bug! As
Thomas B stated, read the manual. Here is a section regarding flying over water:
"
Operate the aircraft with great caution in the following situations:
Flying over monochrome surfaces (e.g. pure black, pure white, pure red, pure green).
Flying over a highly reflective surfaces.
Flying at high speeds of over 22mph (36kph) at 2 meters or over 11mph (18kph) 1 meter.
Flying over water or transparent surfaces.
Flying over moving surfaces or objects.
Flying over inclined surfaces that will deflect sound waves away from the aircraft.
Flying over small and fine objects (e.g. tree branches and power lines).
Flying in an area where the lighting changes frequently or drastically.
Flying over extremely dark (lux < 10) or bright (lux > 100,000) surfaces.
Flying over surfaces that can absorb sound waves (e.g. thick carpet).
Flying over surfaces without clear patterns or texture.
Flying over surfaces with identical repeating patterns or textures (e.g. tiling)."
I've read many a posting on this forum where pilots were flying over water, and then "for no reason" the aircraft went into the water.
The cause? The downward sensors getting confused, causing the aircraft to descend into the water.
Some people have found that it is better for them to turn off the sensors when flying over water.