Thanks to the Pet Journal, two more pets have had their sources revealed! Or maybe these are just placeholder sources and Blizzard is trying to throw us off the trail. :P
According the journal, the Aqua Strider is a BoP drop off of Nalash Verdantis in the Dread Wastes (level 89-90 Pandaria zone).
In the same boat as the Aqua Strider is Grinder, who is also a drop off of an NPC from the Dread Wastes. Grinder is a BoP drop off of Karr the Darkener.
I'm not having any luck finding either NPC in any database at the moment, and I still don't have a level 90 toon on the beta so flying around searching for Nalash and Karr is a no-go for me atm. So unfortunately, I can't confirm this nor can I provide any other details.
Are these NPCs quest mobs? Rarespawns? What are their spawn locations? If anyone has more information on them, please share!