It's dead center across from the MK main entrance. If you exit the park and go out between the 2 bag checks, it will be directly in front of you. If you're coming off a bus or monorail, walk down the hill towards the center bag checks, and it will be opposite them.
In this snip from Google Maps, the MK entrance is at the top; the white inverted "V" at the center is the bag check, and the blue inverted "T" at the bottom center is the dock for FtW & WL. The monorail station is the large white/grey structure on the left, and the bus loops & ferry terminal are at the right.
View attachment 85694
This seems to be showing the ferry dock at TTC, not the MK boat docks,