How do bodies of water hold heat?

Technically they don't hold heat - heat is only energy in transit between two masses due to a temperature difference just as rain is only rain while it is falling (once it hits the ground it is a puddle or a lake or a river, etc.) What bodies of water hold is thermal energy, i.e internal energy that can be transferred if the water comes in contact with something at a lower temperature. Energy can be transferred into bodies of water (via heat) by conduction from warmer substances or convection as warmer substances flow into them or by radiation (like the sun shining on them). The bodies of water absorb the heat and warm up with the energy being stored in the molecules of water in the form of increased vibration of the OH bonds, rotation of the molecules and translational energy as the molecules rattle around banging into each other.