Both the monorail and boat are great. The monorail is probably faster in general, but keep your eye out as you approach the area. You basically walk right past the boat launch on your way to monorail ramp. Be certain to go to the RESORT monorail, not theone to the TTC!
Anyhow, as you are walking up to the ramp, if you happen to see the boat approaching the dock, then that's a pretty good choice. If it's leaving or you don't see the boat, then the monorail is probably faster. The boat generally takes a little longer, esp since it stops at GF first...but there are times when the monorail stops running temporarily.
The boat ride is a nice quiet break from the noise of MK...but there's something special about arriving to MK via monorail!