Can't say enough good things about the Baltic Jewels & the Midnight Sun cruise. The ship was fabulous, the cabins were very nice, the food was very good, entertainment was very good, service was top notch.
We went from about mid-May to Mid-June and the weather was quite good. As far as rough seas goes we only had a couple of days where it as a little bumpy, out of almost a month on the ship. The first was traveling from Denmark to Bergen, Norway at night (just pretend you are in a water bed in the 1970s!). The other was traveling from Norway to the northern islands in Scotland. I don't recall seeing anyone that appeared sea sick on the trip. The newer ships are very good at keeping things as stable as possible.
Probably the only possible negative is that it's not cheap, a couple of the guides (out of many) could have been better and our excursions got messed up in their computer system between our first 2 weeks (Baltic cruise) and our second 2 weeks (Midnight Sun cruise). Hopefully, they have these things ironed out now.
I would avoid any cabins like ours at the back end of the 6th floor if you are taking a cruise in Norway. The problem is that the 7th floor flares out from about the middle of the ship back to give more seating for the dining area. This causes a problem in the Fjords because it blocks the upward view from your balcony. You have to go to the public area which defeats the purpose of having a balcony. Just keep that in mind.
The cabin itself was very nice.