I hope all you're doing in the bathtub is taking a bath. I mean, you can do other things if you want I guess. But bathtubs were made just for baths!
When do you miss taking a bath the most? When you're on the road (ろ). Traveling is fun and all, but when you're stuck in a car, traveling down a dirt road for days, all you want to do is take a nice long bath.
Currently there are no similar looking kanji which exist in the WaniKani learning system