Brackish water which is fresh water that meats salty sea water may be safe to drink if it's your only option and you have no other water to drink.
However the brackish water could contain bad bacteria as well that could cause death or severe illness so I would not advise anyone to drink brackish water if they have other options.
If you can it's best to drink water out of a creek or pond as that water would be safer for you than any brackish water would be that comes from an unknown source.
The brackish water could be too salty as well which could actually dehydrate you more than hydrate you.
But if it's your only option then drinking brackish water may or may not hurt you but if your gonna die of dehydration anyway then it's worth a try.
Me if I have other water sources available I will avoid drinking brackish water just for my health and safety as it's not worth the health risk.