If you have an Apple Watch Ultra, with watchOS 10 or higher, you can measure the Water temperature. Choose the function from the Settings tab inside the app, or at the bottom of the list of sports, and put your arm under the water, at least for 30 cm.

When you pull out the watch from the water, you will be able to read the temperature for 3 seconds.

The OS will lock your Apple Watch with the Waterproof drop and you will have to press the Crown for a few seconds, to remove it.