There are 3 types of boats that serve the MK waterways. The largest are the Ferry boats from TTC to MK, these are always wheelchair friendly. The next largest are the Cruisers, there are 3 of these(orignally only 2, one was added 10years or so ago)2 serve the FW to MK route and are generally wheelchair friendly(FW has no other direct route to MK), 1 serves the WL to MK route. The 3 boat is the Launch,these boats require a step or two into them and are not wheelchair friendly. 1 serves the WL to MK route, 2 serve to WL, FW, CR route and 2 to 3(depending on need) serve the Poly, GF, MK route.
With the exception of the newer cruiser The Mermaide, all the other boats have been inservice since the opening of WDW and I doubt anymore boats will be added to the fleet anytime soon. I seem to recall there were originally 7 launches, but one seems to be missing now, I wonder if when it was removed from service(for whatever reason) that that is when the commissioned the new cruiser.