The Vanguardia has a table on the sources of petroleum pollution in the oceans. In thousands of tons of petroleum and its derivatives spilled into the oceans per year, these are the six biggest sources:

Sewers (urban and industrial waste): 1,343.
Ship maintenance (cleaning, etc.): 466.
Atmospheric emissions (carried by rain to the sea): 340.
Natural sources (undersea geological releases): 229.
Tanker accidents: 126.
Oil drilling platforms (maintenance and accidental spills): 51.

What the chart shows is that tanker spills have a disastrous effect but only in small, specific places. The best thing that can be done to protect the environment in general from petroleum pollution is to build water treatment plants and hazardous waste disposal sites and to stop dumping the stuff straight from the sewers into the ocean. This kind of pollution is concentrated in poor countries, since the rich ones have already built the necessary facilities. Barcelona has barely started on its water treatment plants; they've built a small one on the Llobregat south of town, but they have to build another to take care of all the crap from the Barcelona suburbs out there. As for the Besós north of town, they're building a big plant right now which will go into operation within a couple of years. But, right now, of the crap that Catalonia dumps into the Mediterranean, only about a fourth of it is treated; this hasn't been permitted in America since the Seventies. The other thing that really needs to be done is some enforcement of the international maritime standards on when and where you can clean the bilge out of your ship. Again, this kind of pollution tends to be concentrated in the Third World--the Equatorial Guinea harbor police, say, probably aren't nearly as efficient, or existent, as those in Copenhagen. After that, doing things to reduce emissions into the atmosphere, like mandating unleaded gas and getting cars with primitive, i.e. pre-Nineties in America, emissions systems off the road is important not only for air quality but for water quality, too.

If I were to put my anti-pollution money where my mouth is, the first thing I'd do is get rid of my 1988 Renault and buy a new car with a catalytic converter. (The Spanish government runs a successful plan every few years to get old hunks of junk off the road, giving you a big tax break if you buy a new car and junk one that is more than, say, ten years old. They also have a strict vehicle-inspection program, and crappy old cars just don't pass it. We applaud both measures.) The second thing I'd do is demand that my taxes be raised in order to build bigger, better water treatment plants so that my poo will no longer just float on out to the Mediterranean. Well, I'm all for spending lots of my tax money on water treatment plants. That should be a major governmental priority. I'm just against spending it on some of the other dumb stuff they currently spend it on--not so much the conservative central government, which has balanced its budget three years in a row, but the Catalanist regional and Socialist municipal governments, neither of whom even bother to pretend not to be lavishly spending our money on toys.