I visited in Riga (Latvia) with my daughter in the beginning of April. Unfortunately the weather was quite bad for any outdoor activity (+2 celcius and rainy), and propably the SUP season is not started in Latvia either, so we were not able to do any SUP there. What was noticed was that in the city center there is a very nice canal which seemed to be very suitable for SUP. According the internet you can hire a SUP board for a canal and a river Daugava paddling in Riga in the summertime.
One location perfect for sea activities according travel books, maps and a view from the airplane is Jurmala (20 mins from Riga by train). Jurmala is by the sea, in Gulf of Riga, and having a long beautiful sandy beach. Maybe next time in Latvia I am able to paddle these SUP places.