Let's think about it for a moment, mathematically. Every single time you pee, you have to flush the toilet. Whereas those foul smelly poops do need considerable amounts of water to send them off to the mysterious aquatic underworld below, urine barely needs any encouragement once the flushing begins. Although many toilet flushes can be stopped part of the way through their noisy waterfalls, most can't, and that's wasting a heck of a lot of water.
An average flush for a modern, Western-style toilet uses 6 liters (1.6 gallons) of water, and the average adult pees about seven times every 24 hours. That means that each day of weeing takes 42 liters (11.1 gallons) of toilet water to flush away. Assuming that people urinate the same way every single day, this means in just one year, the average person uses 15,330 liters (4,050 gallons) of toilet water.
There are 319 million people in the US, so assuming they all pee the same which they don't, but we're making broadly accurate assumptions here that's about 4.9 trillion liters (1.3 trillion gallons) of water flushed away in the name of urine every 365 days. For comparison, that's like flushing away 1.97 million Olympic-sized swimming pools' worth per annum.
Now, if you peed in the shower, you'd massively reduce this figure. Say that this means that you only pee in a toilet six, not seven times, per day. This means that in just one year, you'd save 2,190 liters (579 gallons) of toilet water. Extrapolating this to the entire US population, that's 699 billion liters (185 billion gallons) of water saved.
Partly thanks to man-made climate change, and partly thanks to inefficient use of water resources, water supplies around the world are running incredibly low. So go on, do the planet a favor pee as you shower.