Unless it's extremely strong morning or chem piss, it's likely safe to drink. The actual usable liquid (water) in piss will be slightly less than the total quantity since your body will filter out a lot of the waste from the piss, there is still plenty of water for your body to absorb. You will certainly need food, but for hydration, piss should be perfectly fine for that. On the rare opportunities that I've had to serve as a urinal in the past, I do know I pissed a lot more frequently and never felt "dehydrated". If anything, I felt bloated by the large quantities of liquid. Similar to how I get when I drink iced tea on a hot day like it's going out of style.