The warmest ocean water tends to occur in late July or early August. The explanation: While you might assume that the hottest part of summer would occur right around the Summer Solstice (in mid-June when the sun is at its highest point above the horizon and the hours of sunlight per day are at their peak), in most years the hottest afternoons actually occur several weeks later. Why? The earth's surface absorbs more and more solar energy as we move through April, May and June. Then, in late June, after the Summer Solstice passes, the daylight hours begin to shorten and less energy from the sun is absorbed. But the ocean still holds on to some of the energy it received earlier in the summer and cannot radiate, or lose all of it at once. As a result, the warmest ocean temperatures on average do not occur until late in the summer.
By the way, it takes solid surfaces like rock, dirt and sand, less time to get rid of the excess heat stored up over the previous months. As a result, the hottest average air temperatures occur toward the end of July and the very beginning of August, before the warmest ocean temperatures.