I've only encountered this situation once with my
EVO II Pro RTK V2 and it was due to battery exhaustion. I always charge my batteries the night before any flight.
I've always set my first low battery warning at 25% and the critical at 15%, this method has always worked for me from day one. I've used this setting with confidence especially when mapping large sites as once I reach 25%, I'll have enough battery left to return home. Many times if I'm concerned at the distance from my home point, I've also hit the RTH even at 30%.
However, I was experimenting with various warning levels one day in a large pasture and intentionally set the low warning to 15% and the critical at 5%. I was flying close to my home point in case something went wrong out of safety concerns for the aircraft. The aircraft was operated no further than 500' away.
I didn't have any problems when the low battery level hit 15% and the RTH warning sequence started, but as I was intentionally flying to and below the critical level of 5% by canceling the RTH, the aircraft began to behave in a very erratic way. I was no more than about 50' from the home point when the automatic landing sequence started. I was at 4% battery. The aircraft really had a mind of it's own and tried to land but apparently it didn't recognize a good spot. The field was with short grass eaten by cows and was like a carpet.
It was about 5' off the ground when it suddenly quit and dropped to the ground. Thankfully, there was no damage to the craft other than some scruffed up props but I learned not to fly at or near the critical battery warning level. I'm thinking the software performed as designed, however the extremely low battery level probably had also contributed to the crash.
Experiment successful, I reset the battery management at 25% low warning and 15% critical level since then. So this may have caused your craft to land in the pool. You didn't mention what the battery level was, however you may learn something from my experiment.