What is the temperature of water in an underwater cave?

Answer 1

Roughly, a cave's temperature is the yearly average surface temperature of that area. Exceptions: some caves have areas that trap cold winter air and hold that colder temperature into late spring or even summer. Some caves are heated by hot springs.

Answer 2

There isn't a "The temperature". It will depend on the cave and its location for a start.

In caves taking rivers it will be a function of the mean temperature of the surface water the cave is absorbing, and of the air above the water in the cave.

In caves entirely drowned such as those of the Floridan and Bahamanian 'Blue Holes' it will be a function of the mean temperatures of both the sea and the input fresh water.