Water Strider Doesn't Walk on Water

With the patch 8.2 for Rise of Azshara the Water Strider mount will no longer walk on water. 

To walk on water with a mount, you can now use a mount equipment such as the Anglers' Water Striders. You can equip this item in the "Enhance your mounts with Mount Equipment" slot under your mount list.

For more information, please read the Rise of Azshara Content Update Notes and the Rise of Azshara Survival Guide official video.