I like to tell people that Porto (or Oporto) is probably the best SUP destination in Mainland Europe. Easy and cheap to access from almost any European major city (and many from other continents). The airport is just 15 minutes from the city center (and accessible by tram), we have a wide and safe beach for SUP in the city (and it's where our SUP Center is), beaches with less crowd and perfect waves just 15 minutes away, perfect downwind runs that can extend indefinitly (our coast faces west and the predominant wind in the afternoon is from the North (the famous "nortada") with N/NW swells, and rivers, lots of rivers: the Douro River that crosses the city and its World Heritage location and its numerous affluents (Inha, Paiva, Sousa, Arda), where you can peacefully paddle in the middle of the Nature .
Here's a video of a one of those tours: