Water is an excellent medium for storing thermal energy due to the following reasons:
1. Water has a very high sensible and latent heat capacity - suppose you had a choice of storing an amount of sensible heat at a given temperature in either water or concrete - you'd need almost five times as much concrete by weight which would consume around two and a half times the volume that water would otherwise require.
2. Heat can travel in and out of water easily compared with other materials meaning domestic radiators and automotive cooling systems are reasonably small