You will have no reason to get on a boat, but if you want to:
Take a bus to the TTC and ride the ferry across Bay Lake to get to the MK instead of the monorail.
or park hop to Epcot from MK by taking the ferry and then the monorail
(The MK ferry is huge, but is often faster than the monorail).
Take the boat between Epcot and MGM. Its a slow boat, but its a nice ride and you'll get a peek at the Epcot area resorts. The fastest way between MGM and Epcot is to walk (it is quite a hike, about a mile, but the boats make a lot of stops and the bus has to go the long way around), but if you are near the International Gateway, taking a boat is better than walking to the front of the park for a bus.
Take the resort boats around Bay Lake. If you want to eat at Whispering Canyon or Trails End, this is a great way to get there from the MK.
Go for a ride on the Steamship at the MK.
You may want to note that the Friendship Boats from MGM to Epcot, the big Ferry, and the Resort Ferrys are three very different kind of boats. I think the resort ferrys are the most fun, but they will also be the most out of your way unless you need to visit a MK resort.