My wife and I are heading back for our 4th trip to Hawaii in 4 years in Jan as well.
For the spinner dolphins, we found the Big Island was the best spot. We tried Oahu but found they were too restrictive with flotation devices being worn in the water. On the Big Island we went with Coral Reef Snorkel Adventures. It's a private boat for about 5-7 people which gives you the freedom with do whatever you want. Be it spend the whole morning with the dolphins or go whale watching.
Some of my pics from our dolphin trip are here: http://www.danielbus...pinner-Dolphins
For snorkelling, you can do the tours or just drive yourself to the Captain Cook memorial park. We've seen a couple of small white tips there but because there are so many tourists and boats you probably won't see much.
If time is an issue I recommend some shore dives. We've done a few on Maui on the north coast and it was great. If you do the old jetty at Lahaina you'll find quite a few sharks and turtles like here:
Monk Seals - I've only ever seen one and that was on Kauai. Good luck with that.