The cruise ships typically give you a several-hour sniff of a few locales: Honolulu, Hilo and/or Kona, Lahaina or sometimes Kahului, Lihue. In a place like Lahaina, that means you can see the banyan trees, buy some trinkets and that's about it. If you get something approaching a day in Honolulu you can get over to Pearl. In Kona they might drive you to a tourist-trap coffee "plantation." This is not necessarily an indictment, btw, and shore excursions do vary.
If you like the cruise aspect of a cruise, then cruise. If you want to see Hawai'i, fly over. I know you've hinted at financial constraints, but not knowing what that means, I'm going to suggest a "land" trip ... unless you really like the cruise part over/back. Hawai'i need not be $600 hotel rooms and $35 breakfast buffets, and rental cars are dirt dirt dirt cheap over there.
Whales are a late-winter/spring thing. Two of the best areas are the west side of Maui and the north shore of Kauai. You should see some in transit but it won't be like a dedicated small-boat whale excursion.
As for a land trip, I think island hopping is a mistake unless you have several weeks. Hawai'i goers all have their fave islands, and reasons why. I think the BI is the "best" because of its size (the same as the other islands combined) and diversity: from near-desert of Kohala to the active volcano to alpine to tropical. Terri prefers Kauai for the lack of crowds, and it's easy to get from the dry side to the wet side so as to experience it all. Maui has certain things going for it, particularly from a food perspective (though the food rocks throughout). I'm not a fan of Oahu, unless you're a north shore surfer and of course I think Pearl is a must-do ... all of it, not just the AZ Memorial. But some folks want the urban vibe of Waikiki. Molokai is for those who really want to get away from it all. You really can't go wrong. If you are talking a week or 10 days, I'd do one. But that's just how we travel; we don't like experiencing things in an if-this-is-Tuesday-this-must-be-Chicago fashion.
What type of activities are you interested in? I mean, you can snorkel or play golf on any island, but I think there is quite a bit of difference in see/do among them all.