Might be a bug; I can reproduce it but I do not know if it can be reproduced by other players on other server types.
While riding a horse, I jumped over a rock and into the newb river. The horse and I both went under water. I could move the horse right and left, but up command was not bring us above water. The horse’s head did come above water when I was close enough to land that it could put the feet to the ground.
After getting the head above water line, I still had the breath bar and it was not recovering. Once we both were out of the water, the bar faded off.
Shouldn’t the up button return the mount and rider to the surface?