The best place to ride on Oahu is Keehi Lagoon at the birm but you need to be a member, or be with someone who's a member to use the docks at that site. Most of the people in the clubs are really chill and it wouldn't be tough to hook up with a few of them once you're over there.
The website for the OWSC is www.oahuwaterskiclub.com
. There's also 2 other clubs called Na Alii and Hawaii Water Ski.
The site has a jump, 6 ball slalom course and a rainbow slider.
However, because of the recent Tsunami events, some damage has been done to the clubhouses and docks which will be repaired over the next month or so.
Some people also launch their boat in Kaneohe Bay, but you have to get lucky with the wind to get good water, or just go early.
Don't bother with Koko Marina in Hawaii Kai. The rules are strict (can't ballast boats and must drive in circles) so the water isn't great for riding. Plus with all the commercial boats going in and out all the time, it's always rolly.
You can't ride at Ala Moana or Sand Island. I hope this helps. If you have any other questions, let me know. Otherwise, have a great move over to the islands!