This oil was pretty light overall. And the Gulf is a huge body of warm water with microbes that actively eat oil seeps, as opposed to Prince Edward Sound, a restricted and cold body of water. It can absorb and consume this oil.
It can be light crude, yet still contain heavy molecules.
My specialty for the past 25 years has been the heavy fractions many of which are heavier than water.
If oil is 10 API or lighter, it will float in fresh water.
If oil is 7 API or heavier, it will sink in salt water.