Hi there!
You may have some problems with buying a drysuit, especially in February. Most shops located in Gdynia, Sopot or Gdansk are, let's say, season-shops. It means that they sell winter stuff (snowboards, skis, jackets) during winter and summer stuff (kites, boards, clothes, etc.) during summer, but I don't know exactly when they change.. (probably something like April).
You may also have difficulties with finding a drysuit, because, as far as I know, vast majority of suits are wetsuits. Honestly I have never seen a drysuit, maybe because I wasn't looking for it...

But don't worry, thick wetsuit, even for thin person, is sufficent even in April and if you are equipped with hood, boots and gloves, you'll surely fill warm and comfortable. Of course some may say that they don't need boots etc. but I don't belong to them
Here you have the list of shops that I know:
- Huragan (Hurricane) - Gdańsk, Targ Rybny Street, 6,
- ABC Windsurfing - Gdynia, Aleja Zwycięstwa 190 (close to Klif mall)
- Locales - Gdynia, Wiejska Street, 129.
Good luck and regards!