Can you get water temperature only while swimming?
Can get water temperature on the watch while swimming?
Apple Watch Series 8 and Apple Watch Ultra have 2 temperature sensors—one on the back crystal, near your skin, and another just under the display. While you sleep, Apple Watch samples your temperature every five seconds. The temperature sensing feature is not a thermometer and cannot provide wrist temperature measurements on-demand. For more info, see:
Track your nightly wrist temperature changes with Apple Watch - Apple Support