Will this keep my sensor dry while doing water sports, like snorkeling?

i do not believe that they keep your sensor dry. It’s adhesive can and does withstand showering. I use SkinTac before applying to give the sensor and patch better adherence because when swimming it tended to begin to loosen. The SkinTac helped with that greatly. I have an inground pool and spend a great deal of time in it but remain mindful of the libre sensor ( if that is what you refer to ) having a 30 minute restriction on being in water. Then it must dry completely before getting wet again. I hope this helps It is a very good product.

Will these work with the Dexcom G7?

These are just the right diameter to work with most "round" CGM. I believe the Dexcom G7s shape wouldn't work well, for 7 to 14 days. I'm sure it'd be ok short term but theses have good adhesive. They last better than the ones I receive with my CGM.

Hi. The diameter of our freestyle libre adhesive patches is 3 1/2".

can 1 person apply these to back of the arm easily

Yes, provided that the sensor is not placed too far back on the arm. While a second person can bee helpful. I frequently apply apply both the sensor and the patch by myself