OR it indicates that marine fossils are denser because they LIVE in water and do not float to the surface like vertebrates tend to do.
What makes you think that is true. You do not get to make stuff up as an answer. You have to think things through. Remember that fish are "vertegbrates" too. I think if you check you will find that fish and we are about the same density and both when dead float for a bit after rotting sets in and then sink.
Your explanation doesn't hold water.