Efficiency sucks, waveform sucks, grounding sucks, extremely useful.
Depending on what you mean by 'water boiler', an appropriately rated inverter will work fine. A water heater designed to run off 12/24V would be a bit more efficient, of course, but might be unobtanium. I'm hoping you mean the sort of thing one might dangle in a mug of water to make it hot, rather than an industrial tea urn
[1] or something. There's a limit to how much current the alternator will supply - a substantial heating load is likely to exceed it.
If you're really serious about making hot water using vehicle power, you might be better off tapping into the engine coolant circuit and running a heat-exchanger. Or writing it all off as a bad job and using gas like normal people.
Either way, a >300W inverter is an absolutely wonderful thing to own. It'll run all manner of random electronics in random situations, including power tools
[3].
[1] I have a long-standing hatred of tea urns. Apart from obviously not liking tea, they've been my electrical arch nemesis on a number of occasions[2].
[2] The sort of occasions where you're running a ring main at near its rated load in order to run a lighting/sound rig. This invariably works fine during rehearsals, and then on the opening night someone plugs an unanticipated tea urn in halfway through the first act and trips the breaker. With hilarious consequences.
[3] Top tip: It's a good idea to pay attention to grounding when using an inverter to power a soldering iron in order to work on the vehicle's electrics. Don't ask how I know this.