Some great answers here--and some entertaining ones that have kept me laughing!
It doesn't need to be Titanic-level dramatic, fortunately, as the story's only short (or at least that's the idea! Sometimes they can sprawl...)
I've been doing a bit of research while checking here for responses and another boat-sinking possibility has reared it's head: I read somewhere that if a weed hatch isn't fitted back properly then water can get into the stern/engine bay that way, especially if the engine is running but presumably also if the wind is brutal? Does that sound more/less feasible than what we have already?
Assuming that water does start getting in, either through a hole in the hull, the weed hatch, or whatever... is it reasonable enough that a boat could end up on the bottom of the canal over the span of an hour or two? Again, assuming the weather is poor.