When you think of snorkeling on Oahu, most people tend to think of Hanauma Bay, the beautiful nature preserve on the eastern side of the island. However, there's a beautiful spot on the Western side of the island just north of the Ko Olina resort area.
Kahe Point Beach Park is also known as Electric Beach because of the electric power plant located across the street. The outflow of clean warm water from the plant attracts lots of fish and sea life and the water is clear here. There are no lifeguards on duty here and it's best for intermediate/advanced snorkelers since there is a swim involved and the water gets progressively deeper. Do not dive down in front of the opening of the pipes!
If you're more of a beginning snorkeler, you can drive a little further north down the road past the power plant. You won't be right in front of the warm water outflow pipes but you will still see a lot of fish and sea life. Alternatively, there are snorkeling tours that will take you to this area too.