I'd have a hard time rationalizing the difference, too. Is this an indoor or outdoor pool? Could you be feeling the extra heat from the sun? Also, did you measure the temperature? How was the ocean temperature measured? These are all things I'd want to check before guessing.

Typically, when things of the same temperature feel different to the touch, it's due to their different thermal conductivities. For example, a piece of metal at 75 degrees F will feel colder than a piece of plastic at 75 degrees F. This is because the metal is more thermally conductive, which means that heat travels through it more easily. Therefore, the metal draws the heat away from your body more easily than the plastic does (plastic is not a good thermal conductor, which is why it's used on the handles of cooking pots).

When I was a kid, I'd often go on hiking trips with friends, and we'd wrap our sandwiches in aluminum foil thinking that it would keep them colder. It wasn't until I learned about thermal conductivity that I realized that the aluminum probably didn't do any good - it just felt colder.