Inductance is only relevant for the maintenance of an arc with fluctuating impedance when dealing with DC. It's why old transformer-type DC welders and carbon arc power spplues used big chokes in series.

Beyond that, 30VDC is 30VDC. Two or three car batteries in series will sustain an arc.

Switching DC is no joke. That's why cars and trucks run on 12 or 24VDC, instead of 72 or 90VDC like locomotives - DC equipment gets much more bulky and expensive at higher voltages. E.G. a contactor rated for 40 horsepower at 600VAC will probably only handle 2-5 horsepower at it's maximum DC rating of 90~120VDC... if it even *has* a DC rating at all.

Respectfully - you have overshot the point, switching a 48 volt contactor coil that draws only 1A is next to trivial for a mains rated thermostat on a water heater ( these are usually >= 10A ac ).
In fact the inductance of the line whether welder or otherwise serves to establish AND maintain the arc for DC systems - this is why we have to have an idea of the L in the line when designing breakers for an application.
2 or 3 car batteries in series will not sustain an arc if the series load ( read contactor coil ) limits the current to 1A - you will get a very small and very very temporary arc that will be hard to discern in daylight.
Posting worst case short circuit scenarios borders on the unhelpful

 

We're on different pages. I'm saying it's a stupid idea to switch 48 volt DC load current using using an AC-rated thermostat. I'm not talking about pilot current here dude. Anything will switch pilot current.

Stuff a mercury relay or SSR into it and be done.

 

I got a reply from the chaps at oemheaters.com:

We can make this work on 48V up to 1500W. We’d add an external sensor to the solistat that could either go in a thermowell or into the tank directly. You’d just need a set of on/off setpoints. (perhaps ON @ 79°C and OFF @ 81°C) – A 2°C differential is required. Price on that would be about $117.00 – Lead time is 2-4 business days right now.

Not cheap, but it looks like this might be a simple one-component solution.