The Water Tablet. A sacred artifact that enables you to walk on water. Now, as me and my cousins have thought before-hand, Amaterasu would beat Orca in getting across the seawater faster. Were we terribly wrong. We actually timed the two since Ammy seemed much slower, but we couldn't believe it. Orca took less than 10 seconds to reach one of the islands in Ryoshima Coast, while Ammy took around an approximate 25. So whenever you return to the coast to:
1 - Get the treasures hidden inside the pink clams underwater, found on rocks in the sea. Plus, by the time you've gotten the tablet, the Water Dragon is gone, so it would be more ideal to go get them now.
2 - Get to the several islands in order to get secrets among other things.
3 - Re-visit places like Catcall Tower for maybe some unfinished work perhaps.
Whatever you do, I recommend using Orca instead of Amaterasu as you save more time that way, and have less frustration. I would be one to know from not noticing sooner.