You see a turtle.
Location
The shores of Meriana and the Laguna Islands.
Notes
They seem to be treated as pets by mermaids, and can be used for transportation between islands that do not have normal seaports. Simply stepping onto a sea turtle will cause it to transport you to a nearby island, but only if you have permission to use them. Sea turtle transports are protection zones. Throwing lootbags on the turtle won't work, you will have to use the royal mailbox on the Laguna Islands instead.
After having travelled on the back of one of these turtles 2000 times, you will receive the achievement Quick as a Turtle.