You can buy Darkwater Skate from Galissa Sundew(around Darkmoon Island) with 500*Darkmoon Daggermaw.

The Location of Galissa Sundew in Darkmoon Island:

Darkwater Skate is a new aquatic mount added in patch 7.1, and it is also a new Darkmoon Faire mount with the exception of Darkmoon Dancing Bear and Swift Forest Strider.
Darkmoon Faire event will come on the first Sunday of each month, which lasts for one week. You can get some Darkmoon Prize Tickets or some toys by completing missions. The tickets can be used to exchange for mounts, pets, toys, heirlooms and transmog gears.
We can buy Darkwater Skate from Galissa Sundew(around Darkmoon Island) with 500*Darkmoon Daggermaw. Darkmoon Daggermaw is a currency which is similar to the Darkmoon Prize Ticket. These are obtained by fishing in Darkmoon Island and is pretty common there. Galissa Sundew will sell some fishing stuff for the Darkmoon Daggermaw like Darkwater Skate, wow pets, toys and so on. Spending 100*Darkmoon Daggermaw can buy Faded Treasure Map and find the mysterious treasure will reward 100*Darkmoon Prize Tickets. That is the fastest way to get Darkmoon Dancing Bear or Swift Forest Strider.
If you have a lot of wow gold, you can buy the Darkmoon Daggermaws at the Auction House. So Darkwater Skate will become very simple and one more mount you will get with 500*Darkmoon Daggermaw when Darkmoon Faire comes.