I'll second the Rinnai.
Replaced our aged Vaillant with a Rinnai last year when we totally refitted the galley.
It's much better, will heat water on the highest setting to near scalding point.
seems to use much less gas than the vaillant.
Dont forget to order the draught diverter as well if you decide to go for one.
If your boat is on the thames be carefull of the regs regarding open flame water heaters. Your old heater may only have passed as it was historic equipment.
I had a big row with corgi man over this but he eventuly gave in as it was original equipment.
Can't be sure but a lot of gas appliance thermocouples are the same. Don't know where you are but round here we have Plumb Centres some having a HRPC boiler and heater spares(heating replacement something or another). Try them or similar boiler spares place or ebay
I installed both the old Paloma and its replacement myself. I also run tests on my pipes everytime I shut down (Gaslow system), however, would not be surprised if British Waterways had their own rules.