Unfortunately, the reality of boat life is that just as with a house, there will always be maintenance to take care of which makes boat ownership akin to having both a car and a house – there’ll be domestic matters that need attending to as well as mechanical.
You must therefore account in your finances for a certain amount of troubleshooting and fixing up every year. Precisely how much this is will very much depend on the state of the boat when you bought it and how much care you take of it.
At a bare minimum the engine will need servicing regularly. Marine engines have surprisingly frequent service demands, unlike modern cars.
For example, the Lister-Petter LPWS4 on my narrowboat required a service every 100 hours of use. At maybe four hours of cruising a day, that meant a service every month! (In reality, I would moor for several days between moving so I probably serviced it three or four times a year).
That service was really just an oil and filter change, along with the fuel filter every 500 hours and the air filter whenever it got disgusting. So it was easy enough to do and most narrowboaters, even if they start out life without ever intending to reach their hands into the oily bits, do end up learning the basics of engine management and care, since doing it yourself will save considerable money as well as time and aggravation trying to get a reliable boat engineer to come and do it for you.
Other things that may need servicing or could go wrong include any diesel-fired water heaters and the onboard water pumps which seem to have notoriously short service lives.