I use a sticky adhesive and some tape I buy from amazon - and I am a water baby. You still may have to replace the outer tape, because it can lose its stickiness, but the sensor usually does not fall off. I read this on a UK forum from someone who went scuba diving with the libre and had no issues with it - these products are essential for me.
Skin-TacTM Adhesive Barrier Wipes 50 count
Smith & Nephew 5466000040 Opsite Flexifix 2 Inch x 11 Yards Transparent Film Dressing
Search for these products on amazon as it won’t let me include the links.
The transparent film dressing lasts forever. I’ve only purchased it once and I still have plenty left after a year or more. Wipe with alcohol, wipe area with the skin-tac, apply libre sensor and then cover with tape (I usually do 2 pieces)… I just went surfing this past week and had to replace the tape because it began to come off, but I just use another skin-tac wipe and put more tape over the area (after peeling the old one off very carefully - help from someone is always nice). Good luck!