Temperatures of oceans and very large lakes are constant because of it's unique water properties

It's large specific heat capacity

Specific heat capacity is defined as the amount of heat that is required to raise the temperature of one gram of a substance one degree Celsius

The 2nd unique property of water that helps it maintain constant temperature is it's heat of vaporization

It can be defined as the amount of energy that is required to convert 1.0g of a substance from a liquid state into a liquid

The 3rd unique property of water is its large heat of fusion

It is the amount of heat that is required to melt 1.0g of a solid into a liquid