my mermaids' tails don't start until well past their hips, at least halfway down to the knee area on a human, then their tails are extended.
But I would agree. Most writer's don't include that stuff because it's just a waste of both the reader and the writer's time. They don't need to know when a character needs to use the washroom unless it is a relevant part of the story.
That's part of the reason I'm wondering so much, my mermaids are designed similarly, and have human butts so once I brought that up in the story I couldn't help but wonder where they were...going. i don't think I'm ever going to address it in the story though.