We've just done the cruise, 20th March to April 4th, out of the 8 sea days only 3 were good enough to feel comfortable around the pool, it was quite cold. Weather on the Islands was OK but not spectacular. We missed Kaua'i as the port authorities wouldn't let us in due to the heavy sea swell (weather was OK just roughish seas). Think twice if you are thinking of visiting Haleakala crater on Maui, the weather above about 5000ft was appalling, less than 100ft visibility, freezing cold and horizontal rain! Back at sea level it was great weather. We were told the crater weather was like this fairly regularly.
Having said all this please do not be put off, the ship is great and we enjoyed the holiday. If we had to change anything we probably would not have so long at sea.