I did two excursions that involved whale watching, and I would recommend both, but had a clear favorite. Regardless of the company, whale watching is an incredible experience and one for all ages!
The first time I went, we went with a company called Dolphin Excursions out of Waianae. This is my one absolute MUST-DO for a visit to Oahu. We went in February, which is during the whale season in Hawaii. We got on Naia, a 32-foot boat captained by Ale, an excellent source of information and entertainment. He made it seem like he was enjoying it as much as we were. The boat was small enough that you could sit over the edge and dangle your feet in the water for an up-close and personal experience.
We started out chasing spinner dolphins and followed them for a while. There were dozens of them swimming right along side our boat. After a while, we went out to find a pod of whales. We spent over an hour following six humpback whales, including one that was extremely active, slapping her fin, diving, etc.
After that, we were taken to an area (I think it was called Turtle Town) to snorkel. They provide the gear and you hop out of the boat to see tropical fish and a half-dozen sea turtles. It was incredible! The trip concludes with a small lunch back on land. There is a professional photographer who accompanies every trip out, leaving you to enjoy the experience as opposed to worrying about your camera. He does an incredible job--I got over 150 pictures of the whales, dolphins, turtles, fish, coast, etc., for $30. Because they are such high-quality photos, I have had several of them blown up to hang on my wall (see photos below).
The second whale-watching tour I went on was in Honolulu (this one was for work). We boarded the Star of Honolulu and set out of the harbor to find some whales. We were out for approximately 2 hours and saw two whales. The boat is so large that once one is spotted, it can take several minutes for the boat to turn in the correct direction for best viewing. It made for a lot of impatience on my part! This company caters more to the lunch/dinner cruise crowd, as opposed to those who want to be as close to the action as possible.