Re: They could
A typical passenger ferry only sits in 5-8 feet of water. You cant drop them much ususally - a matter of a few inches usually or all sorts of breakages happen.
Also have to consider the bottom of the river/runagrounds if they go deeper.
Also may have to rengineer the docks at either end to have passengers get on and off - the access ramps need to be approximately level in rivers - in tides they may have to float to maintain a level. The problem happens then if other passenger ferries also need to use the same docks - obvious hilarity.
The cost of changing to swing bridges (and the disruption they cause) also needs to be factored into the life of the bridge. Swing bridges take years to implement, and cause disruption to traffic on a regular basis. The lifespan of a bridge is longer than a ferry, generally.
Finally, rivers change height too - every few years a flood (rarer in Australia than most places, I am sure) might add feet to the height of the river at times - again causing disruption to service. I do not like taking a boat under a bridge with less than 10 feet clearance for safety and sanity.... passenger ferries might do 10 knots or more down stream - thats a LOT of kinetic energy to hit a bridge from the side....
Scrap the ferries or modify them and fire the procurement team.