There are a lot of small ferries that would be interesting to use because of their loading time, but they all have no mail capacity. That may be intentional, but it makes them less than attractive as long as you can use the much larger Turbine Ships and even make a profit.
The much larger turbine ships do not fit up the smaller rivers and canals, however. Also, if traffic is light, they should make a loss (if they don't, or traffic is too heavy, that is a balance issue).