Ka’anapali Beach: CruiserBruce recommended, Ka’anapali Beach is only 4 miles from Lahaina Harbor & about a $15 one-way cab ride. I think that’s a better optIon than the hourly bus that goes to Whalers Village in the middle of Ka’anapali Beach & is about a 10-15 walk to snorkeling. Car rental agencies are near Ka’anapali Beach & with their limited inventory will generally be more expensive than in Kahului which is 45 minutes away.
On the north end of Ka’anapali Beach by the Sheraton hotel is the closest good snorkeling at Black Rock. It’s a lava point that can provide good snorkeling when the conditions are good. However, heed the warning flags & if red or red/yellow wouldn’t recommend going into the water. The current can be very strong in bad conditions but on good days it’s the best snorkeling near Lahaina.
There is a public bus that is about a 5-minute walk from Lahaina Harbor where your tender takes you. With a ship in port, it could be crowded & you could possibly miss the bus which only goes hourly. The bus departs from the back of the Wharf Cinema Center which is across from the large banyan tree adjacent to the harbor.
Here’s some information on the bus ($2 one-way) but I’d recommend taking a cab instead.
Here’s a link to Lahaina which includes an aerial overview of the town:
You can search online for historical information on the easily walkable town of Lahaina which was the capital of the Hawaiian monarchy & the whaling center of the Pacific.
A comprehensive summary for travelers: http://wikitravel.org/en/Maui