"Marina dove into the ocean depths, her tail slapping the surface dismissively" I want to know if this is comma splice or a sort of correct usage of a clause. I don't think its adjective clause, but if it is, please add explanation why.
This sentence is neither a fragment nor an instance of a comma splice. It is a complete sentence and is called a nominative absolute. If you enter "nominative absolute" into the search box on the upper right top of the page, you will find much more information, if needed.