I know that you're frustrated with the pool length, but just remember, it should not affect your work out results. It uses the accelerometer to calculate your activity, not length of pool.
It shouldn't be measuring the length of the pool other than what you entered prior to the workout. Not sure why it is changing it in your summary
When you start your next swim workout, maybe it will adjust itself. Maybe try recalibrating your watch prior to a swim. Go into the watch app Settings on your phone, then go to privacy/motion and fitness you will see an option to reset calibration data.