How does drinking bad water make you dehydrated?

When too much salt ingested and ends up in the blood, the concentration of the blood increases, causing water from the surrounding tissue to move into the blood to dilute it and eventually move into the kidneys and excreted in excess to remove all the extra salt injested
Saltwater, as in sea water, is more hypertonic than the serum in human blood. Therefore, when excess salt is purged by the kidneys, the requisite amount of water is "pulled" out along with it. This process leads to dehydration.