There's a pod of spinner dolphins that regularly patrols the waters on the leeward side - I usually see them from Makaha to Kahe Point (Electric Beach). Easy to see topside - there's also a snorkel boat that departs from Waianae - and I frequently hear them squeaking and chattering under water. Only once have I been in the water while they passed overhead, which was a lovely sight to behold.
Whale season is from November-May. I believe Maui is a favored destination for them, but I've seen them sounding (at a great distance) here on Oahu from my regular dive boat. One or two buddies have seen a humpback while diving (on the leeward side of Oahu) - true, the boat has to give the whale 100-feet of clearance, as do divers, but the whale itself didn't seem to care about that convention, according to my buddy! I dream of seeing one for myself, but until then, the sound of whale song is certainly a lovely accompaniment to any dive. I've heard them sing while diving off the southern and leeward sides of the island, and it's a beautiful thing. Good luck!