The town to go to from Warnemuende is Rostock, a historical town that has been in the sea trade since the middle ages. Warnemuende is the port of Rostock and is also quite nice. There are beaches- but it will be too cold for any bathing There must be public transport from W.-R. It's not more than 15 km- there must be busses.

Berlin is too far for one day and not really close (like 200 km).

I wish I could go on that journey- it sounds gorgeous! St. Petersburg must be simply great.