Things that totally happened in the early days of the 5 year mission due to Jim and Spock sharing a bathroom:
Jim finding the water warm when opening the tap of the sink to brush his teeth and muttering, with sentiment: “who the hell brushes their teeth with hot water?” One day, he hears Spock answer from his quarters “I the hell do, sir.”
Bones having to treat severe burns once because Jim took a shower after Spock had and didn’t check the temperature. Spock always resets it to Jim’s preferred temperature from then on.
Jim accidentally getting out of the bathroom through Spock’s door after a shower, half naked, dripping and flushed and walking in on Spock meditating. Spock cannot focus on meditation for the next two days.
Spock being able to hear whenever Jim is sick, joining him quietly in the bathroom without saying a word, holding his forehead up, helping him clean and undress and tucking him into bed
Jim never turning on the light when he uses the bathroom in the middle of the night so he doesn’t wake Spock. Most of the times, he hits his head on the cupboard over the sink and swears under his breath, then he can hear the smile in Spock’s voice as he asks “are you alright, captain?” from his quarters. He still never turns on the light.
Spock also never turns on the light, but because he’s much more coordinated Jim will never know.
Jim stealing Spock’s make-up because honestly he has the best stuff out of the whole crew
Spock noticing how long Jim takes to get ready in the morning, especially if he has some important diplomat to meet and wears the dress uniform, so he teaches Jim tips and tricks to do his make-up faster and better
sharing the mirror to shave or do their make-up if they have to get ready quickly because it’s logical
Jim giving himself razor cuts all. the. time. because he gets distracted watching Spock
Spock actually, truly bursting out laughing when one day Jim walks into the bathroom with a pillow crease tattooed across his face and being torn between yelling “dammit, Spock, it’s not funny!” and being too in love for words
basically just slipping in love so so easily over how comfortable and intimate and married they were with each other years before they actually married