Swim through all the fish rings. Find one in a cave between Lemuria and Kang's citadel.
Another in the water between the Hexagon Blitz game and the Old West mountains.
Just off the shore of Manhattan, under the ferry boat
In the English river, close to the waterfall
In a small strait, on the Xandarian side of Asgard.
In a dark cave entrance off the coast in Manhattan.
Between some crumbling buildings off the coast of Manhattan.
In Wakanda, at the top of the waterfall which leads into the ocean.
Under the Xandar highway, inside the vault where Stan is trapped. Once he's freed, swim inside that vault for another fish-ring.
In the old West, follow the Xandarmost waterfalls upward. Where the river starts to go underground is where you'll find this ring.
Swimming through all ten unlocks the Asgardian Skiff.