Of note, however, is that both Posejdon and Marina are not walking distance from anything but the beach and perhaps other seaside hotels / pensions, so you need to rely on taxis / Ubers, public transportation or rent a car to get around. The other two Mercures (Gdańsk Stare Miasto and Gdynia) are in proper city centres.

I stayed at the Mercure Gdynia all year round multiple times (I average 10 stays a year there) and never got anything less than the second-tier room (below Privilege, I believe they are called Superior), and IIRC they all had A/C. I actually never felt too hot or too cold, my only complaints about the lower-level rooms were chipped bathtubs (apparently Superiors didn't get new tubs when refurbished while Privileges did, despite the post-refurb bathroom design being basically the same). But yeah, Privilege rooms and the suites are pretty neat at the Mercure Gdynia, and I seem to get a nicer welcome gift with a higher room category.