The temperature of water holds very stable because of its high specific heat capacity.
Specific heat capacity is a measurement of how much heat has to be added to or released from a substance to change its temperature. Water has a very high specific heat capacity (
This is a huge amount of heat required to make a very small change in temperature. Because of this, it takes a whole lot of heat to warm up the water; but once it is hot, it stays hot for a long time.
Think of heating a pot of soup and stirring it with a metal spoon. If we leave the spoon inside the pot and come back 3 minutes later, the spoon will burn our hands, yet the soup is hardly warmed at all. This is because metal has a very low SHC (so little amounts of heat make big temperature changes) and water has a high SHC.