That is an interesting observation, Colin. There could be a few reasons. I just read about upwelling, thanks to you. So it could be the primary reason, as you mentioned you were on the beach. Upwelling is basically the wind movements (parallel to the shoreline) displacing the warm water on the surface of the ocean, where the cold water wells up to replace it. Combine the effects of this with currents or local seabed heating, you might have felt warmer down than at the top. Do share the location where you experienced this and probably we could figure out more. I hope this answers your question to a degree.
