from my experience, i would risk on a teory (more or less cientifically based). When there's a southeast continued weather pattern (like one week with east - levante - winds) the water gets a little warmer, but it also broughts a little rough onshore seas (not very pleasant at this beaches). When there's a continued western weather pattern (very common in summer, with west winds blowing for 2 weeks), the water will likely by cold, because of a local phenomenum called upwelling (deep cold water rising to surface). Of course, there is a intermediate situation, in the transition weather (when it changes from east to west winds, or vice versa), and this means that water can be cool in the first days of levante and still be warm in the first days of west winds. The local factors of each beach also may contribute but in a small scale. I'm from Olhão, and i know almost every piece of sand in algarve, but i absolutely love beaches like albandeira carvalho and barranquinho, as i love some beautiful small deserted sand spits here in Ria Formosa coast (people from carvoeiro area should visit Ilha do Farol e Ilha Deserta, you would get quite astonished, better if you can rent a boat to visit other desert islands in culatra). The explanation i gave works almost everytme in perfection for sotavento area (spain until albufeira) and influences also much of the barlavento. To my patterns, carvoeiro area sea is a bit cold ( i have been there 5 or 6 times and never get a "normal" water. But don't worry, sometimes here gets also that cold. I think the main difference here is that we get far warmer than carvoeiro area because of the Gulf of Cadiz warm current (exists only in the first situation i told, and it flows westward) in its full power! In winter the situation is quite more stable, because of the influence of the gulf stream brought by antlantic kind of weather, so it doesn't get neither cold neither warm. Just for the record, i have been in slightly warmer sea at Praia de Faro and even Sagres in winter (there isn't pratically any diference) than in summer in Carvoeiro area... blessed Algarve

Here's another view on coldwater. I know it will get thrown off or moved, but I thought with the appropriate heading.............
Can cold water clean dishes?

This is for all the germ conscious folks that worry about using cold water to clean.

John went to visit his ninety year old grandfather in a very secluded, rural area of Georgia.

After spending a great evening chatting the night away, John's grandfather prepared breakfast of bacon, eggs and toast. However, John noticed a film like substance on his plate, and questioned his grandfather asking, "Are these plates clean?"

His grandfather replied, "They're as clean as cold water can get them. Just you go ahead and finish your meal, Sonny!"

For lunch the old man made hamburgers. Again, John was concerned about the plates as his appeared to have tiny specks around the edge that looked like dried egg and asked,
"Are you sure these plates are clean?"

Without looking up the old man said, "I told you before, Sonny, those dishes are as clean as cold water can get them. Now don't you fret, I don't want to hear another word about it!"

Later that afternoon, John was on his way to a nearby town and as he was leaving, his grandfather's dog started to growl, and wouldn't let him pass.
John yelled and said, "Grandfather, your dog won't let me get to my car".

Without diverting his attention from the football game he was watching on TV, the old man shouted "COLDWATER, GO LAY DOWN!!!!"