Researchers have done sonograms of women before and after arousal, with them having used the restroom immediately at the start of the process to control for initial urine levels. The bladder of a squirter fills rapidly with liquid prior to squirting, significantly faster than it does in similar non-arousal conditions. When you pee, you never void all of the urine in the bladder, so there is definitely pee in squirt, but the urea content was significantly lower than in the pre-arousal urine. IIRC, the researchers determined that the added liquid is mostly just water.

So, squirt is pee, but if she’s gone to the bathroom recently, it’s very diluted pee.