I was diving off of Waianae last week, and I think the water temperature was ~80 degrees at about 50-feet.
If you're peeved by the ride out to Waianae - and it can indeed be a royal pain the proverbial, given the traffic - then seek an operator/guide that takes you out of Hawaii Kai. For what it's worth, the dives on both sides of the island are lovely - I don't know if you can say one is 'better' than the other. For example, Fantasy Reef on the south side and Airplane Canyon on the leeward side are among my favorite sites off Oahu, but it's hard to rank one as 'better' than the other. They're both awesome in different ways. That said, the south side has some stellar dives like Sea Cave and China Wall that you'd be hard-pressed to find comparisons with on the leeward side of the island. About the only thing one can definitively say is that the leeward side of the island has more reliably excellent diving conditions, although the south side's pretty darn decent in the winter time.
As for a day trip to another island in order to dive...I wouldn't, if previous posters' information holds true about interisland flights
http://www.scubaboard.com/forums/ha...-flight-kauai-next-morning-will-i-okay-2.html . There used to be a superferry that sailed between Oahu and Maui that would have probably served your purposes better, but it stopped operating some time ago.