So, coffee maniac here - if you have particularly hard water, or if you're using water that's been in an iron storage tank for some time, your coffee is going to be nasty due to the accumulated mineral content.

Water softeners use a process involving ion exchange with tanks of brine; this pulls out the iron and calcium, and makes the water much easier to work with.

If, like me, you have hard water but do not have a water softener, you can put a large pinch of salt in your coffee grounds. It doesn't come out tasting salty, but the quality of the resulting coffee is noticeably better.

The iron storage tanks, as an aside, were quite the innovation for navies when they came into use; prior to that, drinking water had been stored in wooden casks, and was subject to evaporation, algae growth, and other issues. Despite the taste issues, the iron tanks largely solved most of that problem. More modern ships will use stainless, to avoid as much of the metallic flavoring, but IIRC the ability to source stainless water tanks only came about after WWII - so Lionfish's coffee ritual is, in part, a legacy holdover from a prior era.

As a -further- aside, if you use apple-smoked salt in an earthy coffee like, say, Sumatran Mandheling, the result is -utterly fantastic- when you use the dregs of the coffee pot as your working liquid when making chili.