(For certified divers): the best dive shop on Oahu is just a few minutes away in Kailua: Aaron’s Dive Shop. Get with them to set up your dives, or contact them for instruction.
Best instruction on the windward side of Oahu is Twogood Kayaks.
It’s all the rage! Best instruction on the windward side of Oahu Windward Watersports.
Learn to surf! There are many good schools to choose from, the best are: Windward Watersports in Kailua, Hawaii Fire located in Honolulu (have a firefighter for an instructor), and Sunset Suzy’s Surf School, a terrific school on the North Shore.