I had a trial of the libre and it survived 8 swims of under half hour in 2 weeks, as I was trialling it I didn't go over 30 mins till the last swim which was just over 40 minutes. it shallow dived, swam all 4 strokes, tumble turned and only once did I have the 'check sensor in 10 minutes' error, and that was the pool shower not the swim.
I usually swim 1500m breaststroke in around 42 mins twice a week and swim with my 11 yr old a couple of times too, so sulked a bit shortening swims, but went more often instead as the pool/sports centre is just down the road.
Oh and I covered it with a square of Tegaderm.