I've be looking to moor in a place used last time, big enough place, plenty of boats and they allow liveaboards who largely go under the radar.
I agree with the analogy of comparing it to buying a car and whether anyone would choose to borrow money against their home to do this given it's a massively depreciating asset.
I think the maths do stack up, and I'd be worse off by moving to a boat by at least £300 a month. And this is a minimum. It doesn't take into account the opportunity cost of what the £40K might do if it were spent on a bigger house which'll appreciate overtime. Or sticking in funds. I'm putting money away in Vanguard equities and index linked trackers every month and seeing decent returns.
Perhaps it's more prudent to borrow £20K and have a leisure boat. Associated mooring, licencing and running costs will be lower. And it'll be much newer. Used Aintree 25ft boats are around the £25K mark. I could always upgrade in later years to something bigger when more of my home's paid off and the risk becomes less.
I wouldn't have the worry of a potentially not-let property and having to cover the cost of a mortage plus the running of the boat too.
Re. diesel and the future of narrowboats, doesn't look like it's too much of a worry then!