Water today, whether it is tap or bottled (even fresh water sources) is a far cry from what it was 100s of years ago when it ran over rocks and soil picking up all sorts of minerals in solution with a neutral or slightly alkaline pH. Animals including man required this "food" for survival.
The lemon and Sea Salt help to bring the water into a state where it can be better utilized and absorbed by the body.
While in college, I did a paper on acid rain. In northern Europe some of their lakes and ponds had become so acidic that all life in them died and they had become sterile. In an attempt to bring the pH back into alignment they dumped lime into them... but apparently used too much. The larger animal population, particularly the moose were getting sick, and some died. It is all about balance. We have to pay attention to our bodies, listen to them and live/eat/drink accordingly.
as an aside and for comment:
I am beginning to believe that our tonsils are (were as in my case cause my parents had mine removed; wish I had them now) our early warning signal from the body’s immune system. Learn how to listen to the tonsils (among other body parts/functions) physical reaction to your physical state, live/eat/drink accordingly in response and you most likely will live as disease free as possible.