Tongs with thin reins can benefit from the reins being hardened, but generally speaking, I just forge them a little thicker.
Water quench is pretty safe for "mild steel" but don't expect consistent results, as what most people think is mild steel actually is whatever the cheapest steel was that would meet certain minimum requirements, not steel that meets specific carbon and alloying content requirements.
1018 steel won't get appreciably harder in a water quench, a36 (what is most commonly sold as mild steel) might or might not.