WDW monorail capacity is 360 people every 4-5 minutes. I'll even call it 4 just to make you understand how bad it is. Thats 1440 guests per hour. Yes. Thats it. You cannot scale their capacity or the number of trains on the line. Two lines = double that, so lets call it an even 3000. Round numbers are easier.
Buses hold up to roughly 70 guests, lets say there are people not being great and call it 60. So only 6 buses = 1 monorail train. Why would you find it hard to believe that with proper line management, they couldn't get 7 lines that would average dispatching one bus every 4 minutes? Because that is already higher capacity, by 16%. The hold up here would be having the drivers to make this happen. I'm not advocating for replacing everything with buses, even though they are the most flexible and fit the 'peaks and valleys' guest flow the best.
The actual solution is a constant load system, like Skyliner or PeopleMover. While the capacity is still maxed at whatever can run through in an hour and you can't scale, constantly moving lines are always preferred by guests. And getting to the point where you can have a higher capacity than the monorail is easy, provided you have a system that loads onto the constantly moving system fast enough. Do you know what the OHRC for the TTA is? Nearly 5000. Almost 2000 riders per hour higher than the combination capacity of BOTH monorail lines.
The ferries, yes, they fit tons of people, and move them pretty well. Another boat or two would make things even faster, but they'd have to dock them during the day when they're needed.