So many cruises going in that area and as many different companies. Price and quality comes into it and I suggest you read the "Cruise Lines sections to get an idea of which you would like. Weather ...if you reckon on it being similar to an English summer you will probably get the general picture right and the sea conditions should be ok but you must be prepared for the possibility of storms...there were some this year but next year, who knows...other may be more specific but that's my 5p worth....Nei.
I was on a baltic cruise at the end of July this year and unfortunately the weather was like our summer here, cold and wet most days. That said the sea conditions were very calm. Two years ago i was in Copenhagen in August and it was warm and sunny most of the time with the very occasional heavy shower. I think you just have to be prepared for anything weather wise. It's certainly not the destination if sunshine and heat are going to be important to you. The scenery is great and there are many fascinating ports of call so I recommend you follow Neil's advice above about finding the cruise right for you.
My mum went at the end of May and had perfect weather in the 80's every day. You just never can tell..