This applies to all shared boats: one co-owner causes an expensive but uninsured loss, and a (third-party) claimant acts against the richest co-owner, who happened not to be involved in the incident that caused the problem.
Here's an embelishment by way of thought-experiment (*):
One co-owner is unexpectedly unable to use their booked fortnight, and by happenstance, at a barbecue, meets their neighbour's cousin who has experience of lumpy-water cruisers and is really keen on trying a narrowboat, so the co-owner says they can use their weeks. (Or if the agreement between the co-owners doesn't allow subletting, then let's have this neighbour's cousin as a guest on the boat offering helpful tidal-passage advice)
The fortnight includes a trip down the tidal Trent and returning to the canals at Keadby, which is successfully achieved. Sadly Thorne Lock has an unscheduled fortnight's stoppage and the only way to complete the trip on time is via Trent Falls, where a navigational error cutting the corner causes the narrowboat to sink, and a Humber Lifeboat rescue for the crew without personal injury. The wreck is blocking a shipping lane and ABP (the harbour authority) claims its extensive costs in removing the wreck from the co-owner best-placed to pay. The boat was not insured for the tidal Trent below Keadby, and has no value once recovered...
(*) Subletting and scary tidal trips have both happened with other owners on our shared boats, but I've never known of any catastrophic uninsured losses.
I wonder if anyone ever explains to owners signing agreements what "jointly and severally" means, and the thread inspires the question of how US law has adapted this common-law concept. Can't be extradited for debt 🙂 and having assets offshore could in these circumstances be advantageous ...