I am guessing they were custom built. Any number of boat builders around the US could accomplish the task. Plus the boats may have been many years ago. I remember using them 35 years ago. Not saying those weren't replaced over the years, but a well maintained boat can last a long time. So no one may really may know.
think about this a lil more .... WDW operates a bunch of boats and some of them are pretty big, like the ferry from ticket center to MK ....
AND alto they are on lakes, aside from this they are land locked to any other water way .....
WDW has a pretty big boat maintenance facility . . .
here's an annotated map with some points I find interesting ....
factiod .... it is possible for the MK River Boat to be brought all the around to the boatyard for maintenance ... this is why the railroad has a swing bridge just north of Big Thunder ... here you see the River Boat on the other side of the bridge ....
the Magic Kingdom I and Magic Kingdom II, diesel-powered ferry boats, were commissioned and then built at a Tampa Bay, Florida shipyard. Constructed primarily of steel beam and plate, each ferry weighed in at 380,000 pounds. They were put into service at Disney World Resort on July 2, 1972.
The ferries are 120 feet long and nearly 35 feet wide. Their draft is 5’ 6”, which is shallow enough to let them cross the water bridge connecting Bay Lake to Seven Seas Lagoon. Each has two Caterpillar engines that generate up to 400 horsepower apiece. They all have a capacity of 600 guests. The Admiral Joe Fowler can be identified by the green color of the panels down its sides and around its pilothouses. The Richard F. Irvine has red panels, and the General Joe Potter has blue panels.