Alentejo for peace :), and the coastline beachs are great there even to just walk on. On Algarve you can do scuba diving probably easier than other places because of some demand and people doing it there but the stays (and everything else) might be expensive, it will start to get a bit colder from now on pretty much everywhere. Other areas also (like this one pictured) do it but less, i didn't understood if you mean with being experienced or not, i'm really not anyway.
For good prices or longer stays you might want to look at the center or even more inland, places like Leiria, Coimbra, Tomar and there more inland Sertã, Vila de Rei and many more. ;)