If you are around the Hyatt or Marriott at Ka'anapali Beach, there is excellent snorkeling at the end of the beach towards Lahaina. There is a park there called Hanakao'o, and there is a lifeguard station. I would recommend this only for strong snorkelers. Swim out from the beach until you are far enough to clear the rocks on the left. Then turn left and swim down the coast towards Lahaina. The first stretch, there is nothing to see. The water gets clearer the farther you travel. If the water is murky, turn around and go back. The snorkeling will be no good. Go early in the day when there isn't much wave action. If the water is clear, continue swimming until you arrive at beautiful and healthy coral. This is the area, and it is full of beautiful coral, fish, and turtles. Continue down the coast until the coral and fish diminish. Then turn around and repeat the areas on your way back to the beach. This area is highly rated and I experienced it last February. Please keep in mind that it is a long swim, and you can't go directly to shore because it is rocky with wave action against the rocks. You will be rewarded for your efforts.
Also, Kapalua and Honolua Bays are excellent. Go early for parking. Please note that there is a stream that enters Honolua Bay, so if there has been much rain, the water in the bay will be muddy with no snorkeling possible. In this case, Kapalua Bay will be fine.