The boat at MGM goes first to the EPCOT area resorts, and then to the Intl. Gateway entrance to EPCOT. It doesn't go to MK.
The fastest way is to take the bus from MGM to MK.
If time isn't that important to you, you could take the boat to EPCOT, ride some stuff in EPCOT, and work your way to the front of EPCOT, then take the monorail from there to the TTC, then hop on the MK monorail from there. But this is not, and I repeat NOT, a quick way to MK. It's just an option if you want to stop at EPCOT and walk around before heading on to MK.