If you are already on Maui, and then plan to go to Molokai, the ferry is a great way to get there. The Molokai Princess leaves from Lahaina, Maui, and goes to Kaunakakai, Molokai, regularly. The schedule at this writing is:
Leaving Lahaina: Monday-Wednesday-Friday-Saturday at 7:30am and 5:15pm.
Leaving Kaunakakai: Monday to Saturday at 5:45am and daily at 3:00pm.
The trip takes 1.5 hours, and is relaxing and beautiful when the water is calm. Both my channel crossings were smooth, but some of the local folks who take the ferry often said that sometimes it can be very rough. Hold on to your stomachs!
My two favorite parts of the ferry trip were meeting local people (which ended up really helping me when my rental car company failed to leave my rental car at the dock like they were supposed to–-see my entry on "Why Molokai is the Friendly Isle") and whale-watching. Who needs to spend money on a whale-watching trip when you can see dozens of whales from the ferry! Most of the whales I saw were at a distance, but twice the whales surfaced directly next to the boat. On the return trip I also saw dolphins!
On clear days the views of Maui, Lanai, and Molokai from the ferry really knock your socks off. As I was leaving Lahaina, I had a great view back to the wharf and town. The West Maui mountains shot straight up, covered in green, and once again taunting me to hike into them. Toward Lanai, I saw that Lanaihale Peak (3370 feet) was a more humble mountain, roundly curved and not drastically vertical like the West Mauis. Along the coast of Lanai, I also saw Shipwreck Beach, recognizable by the large stationary dark ship. Finally, looking at the south short of Molokai, the mountains and valleys seem to be a combination of the other two island peaks. Steep, with deeply trenched valleys, the Molokai mountains are beautifully colored and interesting to look at. Looking at the south side of Molokai, you have no idea of that the north side holds the steepest sea cliffs (pali) in the world (according to Guinness Book of World Records), the highest at 3300 feet and more than 55 degrees gradient.
The price for a one-way crossing is $40 plus tax (total $42.40). Bring exact change to the ferry dock, or you can pay in advance by credit card. There is usually plenty of room-–it is a good-sized boat-–but you can put your name on a list ahead of time by calling (808)667-6165 or (800)275-6969. For more information, see the ferry website. You should be able to arrange with the rental car companies for a pickup at the ferry, or sometimes they will leave the car and keys if you are coming in late.