I've rerouted using waypoints as best I could to avoid the canals but I am pretty certain that we're going to see a backlog in the canal by Bidstable in short order (it may already be backed up - I haven't logged in today to check). Widening that canal is a possibility but there are several players who would need to participate as each person has docks, rail bridges, road bridges, etc, along that route.
Part of the issue is that the long loading time for ships (2+ hours) causes a significant backlog to occur once we reach a certain level of activity in an area. Unlike with road and rail traffic, which can use platform choose signals to distribute convoys, ships all use the exact same tile to load/unload.
The only way I can see to get around that (somewhat) is to set up multiple parallel lines using different stopping points at the docks. Making those cross-map shipping lines takes quite a while, however, as there are dozens of waypoints in each direction. I may not have a choice, as industrial demand is growing at a significant rate (and will grow even more shortly, as power plants come online).
What is the tile limit for ships? Is there a different limit for loading/unloading operations and transiting through? I assume that the total holds plays a factor in this total. Having 1 vehicle ship convoys would certainly make it much easier. There are no other viable ship alternatives to bulk transport via 8 hold sail ships at the moment.