Every study I’ve read about U.S. waters suggests that the real number for Americans is about 95% of the petroleum in U.S. waters is coming from natural seeps.
The other factor to consider is that catastrophic oil spills are always the result of supertankers dumping millions of gallon in one spot.
When people cite environmental concerns as a reason for not drilling offshore, they’re really thinking about the consequences of NOT drilling offshore, which would keep us dependent on supertanker tranport for decades longer. Offshore rigs ship their petroleum to shore in pipelines which never wreck and can be shut off in hurricanes and so forth, as the lack of any significant spillage during the recent Gulf hurricanes showed.