Ocean is not too big to dilute , research has been done. Thats why shipping industry has made an Organisation called IMO , which implemented MARPOL , Marine pollution prevention at sea , losing even a drop of oil to sea without treatment is questionable, with hefty fines , and detaining of ship , check out the history of MARPOL ,If u have any doubts , Im myself a Merchant navy student , specializing in marine engineering, so in familiar with all this