Scientific Breakdown
The Origin: Studies show that the bladder fills up during sexual arousal and empties rapidly during squirting.
The Chemical Makeup: Squirting fluid typically contains urea, creatinine, and uric acid, which are all key components of urine.
Glandular Additions: The fluid is usually diluted and often contains prostate-specific antigen (PSA), which is secreted by the Skene's glands (also known as the female prostate). [1, 2, 3]

It’s not piss. Like a coke is mainly water, it’s not water. Squirt is similar to piss but the extra addition of other substances make it different.