Zos didn't want to bother with swimming animations for mounts at launch.

However, you will soon be able to buy [crown scroll: mount swimming] for the low, limited time, introductory, specialty price of 10,000 crowns, and if you throw in an extra 5,000 crowns we'll include a one of a kind midget rainbow unicorn donkey mount that leaves pixie dust hoof prints wherever it gallops!

/sarcasm