The fact the Admiral asked this question implies the chances of a shipgirl to meet a guy or even less heard him urinate are zero if not very low or she literally heard him urinate it either by accident or intentionally, another theory is that she just saw it on the internet, i guess when you have the body of a girl after decades of being a hull of steel you start to get curious about the human anatomy, specially the opposite sex. Or also she just remembers her crew when they went to the bathroom, just saying.
Admiral having an evening beer.The sound beer makes as it's pouredsounds like when a guy is peeing.You're saying that to someone having a drink right now.But why does Hamakaze know what sound pee makes when a guy is urinating?