Question

With increasing depth the temprature of sea water decreases to a certain limit

Answer 1

because at the depth of sea water sun lights can't reach so that temp of sea decreases to a certain limit with increasing it's depth

Answer 2

there may be various reasons
1) as the dept increases the rays of sunlight doesn't reaches the bottom of the sea so the bottom ones remains cool as compared to top one
2)the water at the bottom has least velocity or negligible as compared to the top ones
the kinetic energy at bottom is negligible so it does not desipitate the energy loss in making motion