Why are we focused in water balloons when condoms exist? Are they too resilient?
Piss into a condom then squeeze it down into the tip until you have a happy medium between bursting pressure and throwability.
Tie it off like a water balloon, then tie an additional knot (or double knot) a few millimeters higher. Repeat process until you have a bandolier of piss-filled condom balloons.
You can separate them ahead of time or keep it intact depending on situation. Either way, I recommend a styrofoam cooler or other easily disposable hard-walled transport vessel.
This method allows you to customize the caliber of ammunition as well as providing a simple method for multiple contributors.
Every pressure vessel has a burst pressure!