We have been on the Fjords in May & June and had wonderful weather, enjoy.
The chances of rough seas in the fjords are virtually nil. Most ships, even older ones, are stabilised and ride the waves well. If you are uneasy about crossing the North Sea take a pill - I always do. In May or June you may find it a bit chilly on deck if you venture above the arctic circle but in July it should not be a problem (but take some thing wind and shower proof, just in case). Remember the sun screen as you have long days on deck watching the magnificent scenery go by. Have a lovely holiday.