Most of the route is in sheltered waters, so the chances of rough seas are much less. There are a couple of short legs (1-3 hours or so) where you sail in open seas where it could be more choppy. The first night, somewhere between Bergen and Alesund, the ship was rocking gently. I get seasick fairly easily, so I took some Dramamine, and I was fine. It was in the middle of the night and I slept through most of it. The longest stretch of open sea is between Honningsvag and Kirkenes, near the end of the Northbound voyage. It was a little bumpier there, but still not bad. Of course, the sea is unpredictable and there could be storms and some rough water. I think June and July are best the months for weather on the coast - less rain and wind, more clear days, etc. I went in September and we had cloud cover about 50% of the time, and a couple of days of misting rain, but nothing heavy. We were very fortunate to see the Northern Lights, since September is not the prime time for viewing. No Northern Lights in July, obviously, but you'll have the midnight sun and that is very cool. As beautiful as the scenery was in September, I imagine it will be even more lush and green in July! I guess the main disadvantage of traveling in July is just that it is more crowded, but that means there are more interesting people to meet!

I'm attaching a couple of photos so you can see how smooth the water was. The first one is in sheltered water, between the shoreline and some islands. Another benefit of the way these ships hug the coast is the fact that there is beautiful scenery to view on both sides of the ship, so you don't have to worry about booking a cabin on the wrong side. Second photo is when the ship sailed into a fjord, where the water is smooth as glass. I would do this trip again in a heartbeat at any time of the year! Hope that helps. Happy planning!