This is nice work. The only thing I'd change is maybe adding a little more shadow on the smaller dolphin. I think the dancer would cast more of a shadow on that dolphin. Hope this helps.
I think his left hand should be into the sand. I've tried this manuever underwater, and distinctly remember my hand sinking about 1/2 to 1 inch into the sandy bottom. This was in Hawaii, so perhaps this is the Atlantic Ocean where there is a more dense sandy bottom. Perhaps Sponge Bob knows the real answer to this question.