You can go out early and snorkel in the coves right there at the resort.
You can go up to electric beach, but be careful! Do NOT leave anything in your car at all.
You can take a dolphin snorkel boat trip from the Ko Olina marina, which is fun!
We never go to Hanauma Bay from Ko Olina just to snorkel - it's too far. Not that we haven't been there, but not in a long time.