Gdynia has a Baltic coastal climate: cool, often grey, with the marine influence keeping the extremes off. Summer (June–August) is high season — long days, warm enough to swim (water around 18–20°C in July), beaches busy, and the city full of weekenders from Warsaw. September is the city at its best: warm light, school out, and the Gdynia Film Festival in the second week — the most important Polish film event of the year and worth coordinating a visit around if you have any interest.
Winter is cold but not severe, with snow more often dry than deep. The Christmas market on Skwer Kościuszki is small but pleasant. March and April are wet and best avoided.