My personal view is that cold/damp comments like this often refer to being indoors (with inadequately insulated, ventilated or heated properties), and/or at night, and/or in mountain areas, and/or in the 2-3 weeks of bitter cold which happen in some other areas, some years. I say this as someone who still occasionally wraps a blanket around my shoulders when working inside at my desk in April, so I am not saying people are wrong per se!
During the day, most of the year including the winter, in most of the country, it's not "teeth-clutteringly cold" outdoors.
Sea temperatures typically vary by only a few degrees along the whole coastline, as it is all the Atlantic. So, staying in the Algarve may not make much difference compared with, say, the Silver Coast, for sea temperatures.
IPMA, the Portuguese Met Office, has a useful website:
It doesn't easily show historical temperatures, for the sea, although the Notícias section has monthly reports on air temperatures, rain, etc.
Indoor pools for apartment buildings are not common - they exist, but are likely to be extravagantly expensive...
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