You're better off on the boat - just my opinion! The boat takes roughly 20 minutes once it arrives. Whatever time it takes for you to get to the boat dock and for the boat to arrive would be added on.
By car, you could be out of FW in 3 minutes, the drive to the Ticket & Transportation lot is a bit awkward because even though you are so close to the Main Gate (just 1/2 mile to the right from the FW entrance), you actually have to circle around and back which will take you anywhere from 7-15 minutes. Then you park, and either walk or wait for the tram. Add 5+ minutes. Then the Monorail, which takes 10-15 minutes - and that's not including time for it to arrive.
After all is said and done, not including possible delays in one of the forms of transportation arriving, they are probably near a draw. But the boat doesn't require you to bring your car and leave it in a hot parking lot, and the boat ride itself is almost as good as any of the rides in MK - it's peaceful, breezy, and scenic! And it drops you off right at the security search area at the MK entrance.
Bonus for leaving too - the boats are usually less crowded and much easier than searching for your car and driving back.
If you happen to rent the electric cart, then the boat is even better...you just cart yourself down to the boat dock, jump on for the great ride, and when you come back at the end of the day, you get off the boat and jump on your cart for a nice ride home.
BTW - if you really wanted to cheat and find the fastest way in...you could jump in your car and drive to Contemorary, tell the gate attendant you are going to a restaurant inside, park, then walk into MK...total time probably less than 15 minutes. Not that I would encourange that! But the boat ride's the chilled way to go.