While questing in Vash tonight I managed to jump out of the water, and fly around in bird form above the water...
I then dove straight into the water while still in bird form and maintained a speed as though I was mounted on an Abyssal Seahorse, this speed was retained while swimming in caster form, cat form, bear form, while fighting and even after mounting my Abyssal Seahorse.
In fact, this exploit has worked so well that I can't seem to replicate it because I can't find a way to remove this newfound speed.
It could possibly be done by a non-druid class by getting an elixir of water walking and mounting up above the zone and performing the same trick.
Hopefully this isn't a repost but I couldn't seem to see anything similar in my quick search.
Edit: This also appears to have affected my ground mount speed, as I am now traveling at +150% on my Raven Lord...