Why do you suppose the water in the oceans, for example in North Carolina, is still quite cold even during beach week?
The water in the oceans is not stagnant (meaning, they circulate). Therefore, the currents which run through the ocean, in particular the Atlantic Ocean since your question specifically mentions North Carolina, regulate the temperatures of the water.
For example, one of the trade currents which flows through the Atlantic Ocean comes in from the north (near Greenland and the Arctic Circle). The water which comes from the north is rather frigid. While is does heat up a little when it mixes with the current coming from the equator (the Gulf Stream), the water does not immediately heat.
Therefore, the combination of moving water and the frigid northern currents which feed into the Atlantic keeps the temperature much cooler than the waters closer to the equator.