

The meter is defined by the distance light travels in a specific fraction of a second. It may have been initially defined by a rough estimate but it (and all metric measurements) are now fixed to universal constants.
Also the foot is currently defined exclusively as 0.3048 meters, so you are already using metric.

















If you drink something that is about the same ionic concentration as your blood, you can absorb a lot of it safely.
If you drink something much much saltier than your blood it will pull water out of your blood cells.
If you drink something with very few ions it will hydrate your blood cells, but drinking too much can burst your blood cells as they try to absorb to much.