I have a waterproof instant-read kitchen thermometer. Stick the pointy end in the water and read the temperature. Very fast and accurate! I don't usually take it with me on the swim though. Something like this one:

https://www.amazon.com/digital-meat-thermometer-food-thermometer-cooking-thermometer-/dp/B073KYTWGB/ref=mp_s_a_1_3

I like this one. I used it all the time its fast, easy to read and has a magnet so I could stick it on the fridge.

https://www.amazon.com/instant%C3%A1nea-resistencia-calibraci%C3%B3n-parrilladas-adicionales/dp/B073KYTWGB/ref=sr_1_6?crid=36VDJTQSGTKGZ&dchild=1&keywords=digital+instant+read+thermometer&qid=1589931805&sprefix=digital+ins%2Caps%2C224&sr=8-6

High Hydration Bread Recipe

https://youtu.be/tpGlCxqyrLk