Squirt is essentially the milky white from the Skene's Glands combined with "urine".
Take a step back and think about what "urine" is. It's liquid waste mixed with water. But what happens if you drink a ton of water? The ratio of waste to water changes dramatically. Also, you obviously can't maintain the volume yuffie does with normal urine retention. It's essentially a quick fill/release, so very little waste.
If you're unlucky and you get the first round of the day or a bad mix, then it might suck. But overall, don't let the idea of "pee" gross you out, it's mostly water and wont kill you.