You might have already worked this out – but just in case you haven’t – Elixir of Water Walking now works when mounted. You can drink the potion either mounted or dismounted and then use your mount to ride on water! Neat, huh.
Want to get your hands on some? Elixir of Water Walking was previously most commonly obtained as part of the quest reward for the Daily Outland Fishing quests. However as of patch 3.0.8 alchemists (who have discovered the recipe) will be able to make it from Ethereal Oil (learnable from a trainer and requiring Glassfin Minnows).