I have exchanged dozens of hot water tanks. There is a sludge that collects on the bottom. There is also a zinc anode in there that slowly dissolves to protect the steel tank. On older tanks there is nothing left of the anode. I could never drink water from the hot water tank now, personal preference.
I don't find myself waiting too long for water to heat up, perhaps your burner size, equipment profile or methods could be tweaked.
There really isn't a lot of waiting if you plan your brew steps out right. Splitting the total water into mash and sparge helps heat it quicker.
My method: 12 gallon batch size
I start out with the total mash water ONLY in the BK and begin to heat it . As the water is heating, I start milling the grain. Usually by the time that is done the water is at (or close to ) temp. I transfer it to the Mash tun as needed, mashing starts. Then add the total sparge water to the BK and start heating to 180 F while mash is ongoing. When mashing is done, the sparge water is usually ready or within 10 minutes of being ready. Just enough time to bump up the mash to 168 with a little hot sparge water. Lastly, transfer sparge water out of BK at 180 and into HLT for sparging, it cools a bit to about 172. Now sparge can start and can drain into the BK, and once there is a few inches of wort in there; heating for boil starts. Once all runnings are stopped, the wort is almost boiling, and top up if needed at this point to preboil volume.
NOTE: I have a 3 - S.S kettle profile, with a brewpump, raised HLT and 2 propane burners. 15 gal BK, 10 gal MT, 9 gal HLT.
Homemade brewstand from an old york 2000 bench.