Is anything hidden under the sand in the Shipyard?
Solution 1:
I didn't believe Fi either and I searched the sand from corner to corner. Turns out that there is nothing in the sand aside maybe a few hearts and a few green rupees, mostly what you'd expect from cutting grass.
There's really nothing of awesome value in there, so you might as well leave the area... doesn't mean you have to go back to the plot right away though!