I recommend sticking with vinegar and skipping the hydrochloric unless you have some product specifically diluted for water heaters or are a pro with extensive knowledge on use and safety procedures. The muratic/hydrochloric acid sold for pools is so concentrated that it’s downright dangerous. Need to wear acid gloves, goggles, and be really careful. In an OSHA approved setting pretty sure it would require gown, face shield, and ventilation. Putting in too much would also corrode through the metal inside the water heater very quickly, and if any got in your eyes you could become blind and on skin you would get chemical burns.

Vinegar or citric acid (or even glycolic acid which is in Zepps descaler) are weaker acids and are effective at dissolving minerals but gentle enough on the water heater internals so really good for this application. Still be careful not to get any in your eyes.