This will never become a problem.

Take the mediterranean for example, a relatively small ocean. It has a volume of 3.75e15m3, with a mass of 3.75e18kg.

We need 4kJ/kg/K to heat up water. To heat the whole mediterranean by 1K, we need an energy of 1.5e19kJ, or 4.16e6 TWh.

In 2008, total worldwide primary energy consumption was 1.32e5 TWh, 31x less energy than needed to heat the mediterranean by 1K.