To get to the Canaries, you have to cross the Bay of Biscay unless it is a fly cruise. This has a bad reputation, but it is not always rough. It is really not something you can predict. Go for a cabin low and mid ships to mitigate the effect of rough seas. Take seasick tablets with you. Stugeron are good. We have had fairly smooth seas crossing the North Sea so far, but again you cannot tell. But it can be rough in the Med you know! I think our roughest times have been there.
We have crossed the North Sea several times on small ships and have not had a rough crossing yet. he Baltic was a bit bumpy between the Kiel Canal and Stockholm, but nothing to worry about. If you are venturing to St Petersburg in September, remember that they often get the first ground frosts as early as August, so take something warm for evenings.
I We have done 4 transatlantic crossings and all have been good,a few swells, the worst was actually on the IOTS from southampton to the med, luck of the draw I guess, we are doing another transatlantic this October with Princess so hopefully will be ok...Brian.