Right from the Wikipedia page you linked from:
The 10-series receptacle provides 2 hots and a neutral conductor. Grounding is accomplished through the neutral. HOWEVER, if all you have is a ground conductor, you cannot attach a 10-series receptacle to it. Here's why - the ground conductor (see NEC 250.122) may be smaller than what is required to carry the current (even though it's sufficient for fault current) *or* it may be a bare wire conductor. I recognize that for a 30-amp circuit, EGC is the same size (#10); however, if it's a bare wire, then you can't use it on a 10-series receptacle because the grounded conductor (neutral) cannot be bare.
Indeed, 3-wire appliances on a 10-series receptacle *may* ground through the neutral blade, which is why Tesla can sell you a 10-series adapter. However, it is illegal to install a 10-series receptacle onto a cable that has only 2 hots and a ground conductor. You *may* take a cable that formerly supplied a 10-series receptacle, remark the white as green (provided it's a cable and not wire-in-raceway), and use it as a 6-series -- but not the other way around. I recognize the UMC only needs 2 line conductors, but that doesn't make the installation of a 10-30 receptacle legal.
And to your final question, unless the original water heater circuit had a 10-series plug on it, then no, it does not fall under "existing installation"; the water heater was installed with "permanent wiring methods"; installing a receptacle on that is changing it. The same applies to formerly hard-wired cooktops/ranges that required only 3-wire hot-hot-neutral before the early 1990's -- it is illegal to install a 10-series receptacle on that wire for a new range that plugs in -- you must run a new 4-wire circuit to place a receptacle on it.
See my FAQ for more information. Just because it works does not mean it is legal nor safe.
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