If I’d been paying closer attention in Physics 12, I might know the answer to this question. We have a small pool in our backyard, and a small ocean about 10 minutes away. All summer, the Mediterranean has felt warmer than our little pool.
This seems counter-intuitive, because I’d imagine that, over the summer, the sun would heat up the relatively-small amount of water in our pool. The water and surrounding stone would store some of that heat over night, and so would get cumulatively warmer over the summer. That’s happened to some degree, but it’s no match for the ocean.
When I went swimming at Xlendi this morning, the ocean was bathtub warm. What gives?
DarrenBarefoot.com 
No idea about the pool. But one thing is certain… the Med is a sea not an ocean.
Jacques: Er, technically, I think we’re both correct. From Wikipedia:
“The Mediterranean Sea (35 degrees north, 18 degrees east) is a sea of the Atlantic Ocean”
I’d guess a mix of shallows with large Surface Area (to heat) mixed with currents pulling in warm water.
(e.g. how Ireland doesn’t freeze over aka Gulf Stream)
I am with Robin on this one. Another related theory is that if the prevailing wind is onshore then the top surface of the sea which tends to get heated by the sun is pushed into shore and accumulates there. Try swimming on the other side of the island (where the wind would be more offshore) and see if it’s colder.
I suspect currents are the main difference but wonder…
The ocean is salt water and (i’m guessing) your pool isn’t, maybe that’s a factor also.