Me and my brother Pekka were in northern Norway last summer. I would say that the conditions may not be the best in april or may.
We were there in June and the temperature never got higher than 10 C, neither air or water.
Other than that, Norway is quite nice place for spearing. Lots of fish and good visibility.
br,
Tuomo
Edit:
here is the story of our trip:
http://forums.deeperblue.net/showthread.php?t=51298