Looks like there's some truth in it. This snip from http://www.guardian.co.uk/environment/2009/apr/09/shipping-pollution is staggering:
"Confidential data from maritime industry insiders based on engine size and the quality of fuel typically used by ships and cars shows that just 15 of the world's biggest ships may now emit as much pollution as all the world's 760m cars. Low-grade ship bunker fuel (or fuel oil) has up to 2,000 times the sulphur content of diesel fuel used in US and European automobiles."

>I've read a few reports recently that one of the biggest polluters is shipping.
>
>>>>Hi,
>>>>Looks like they are broadly speaking comarable. This claims to show the general picture:
>>>>http://www.grida.no/publications/other/ipcc%5Fsr/?src=/climate/ipcc/aviation/125.htm
>>>>(but it doesn't appear to add weighting to the additional problems of CO2 at high altitude)
>>>>One transatlantic round trip would probably churn out more CO2 per head than most people generate from a year of car use....
>>>
>>>What about the military planes? They are probably the least fuel efficient, they don't serve any useful purpose (other than to take the pilot out for a ride and bring him back), plus they serve a few very damaging purposes - like destruction of other goods or bads, which will then have to be remade, using more resources.
>>
>>I won't argue about the neccessity or otherwise of these flights but, from a practical poluting POV, I'd hazard that they form a miniscule proportion of total air traffic.
>>
>>>
>>>I'm just deurinated for not being able to hear what I say several times a day. They fly over our heads, very low. Ooops, they must be ships, as they belong to the navy.
>>
>>FWIW I probably, at least for the UK, get more than my share of military air traffic - we're in a low-flying fast-jet training area (used by other EU counries as well as the UK) plus, with the SAS just down the road, we get low-flying rotary stuff as well. Not unknown for the windows to get rattled...
>>
>>And there are probably several times a day when others don't want to hear what I'm saying anyway :-{
>>
>>Best,
>>Viv