Sukeytakeitoffagain: emissions from ships are completely shocking, and the health problems are getting worse and worse. According to this a cruise ship emits as much a 5 million cars driven the same distance (can this really be true??!) Marine fuel is v low grade too, and so particularly terrible for health when ships keep their engines running when moored.
Foggy as someone else said, this isn't true:
"Modern Diesel engines (and city diesel) mean modern diesel cars aren't so much of an issue as lorries, trains, vans and busses which can have very large particulate emissions."
Trains ARE a surprising problem (they're sneaky, like ships - not seen as 'bad' and not as regulated), but lorries, vans, buses are FAR better than cars.
In terms of pollution emitted per litre of fuel consumed, a brand new diesel car emits ten times what a brand new lorry/bus emits. (The lorry/bus consumes twice the fuel per km, so it's only 5 times as much per km.)
My own view is that if you live in the countryside, and do mostly rural driving, then diesel is clearly the best option. But if you live in a city, I think that you should be thinking very hard about whether a diesel car is a good investment. It may be ok for a bit, and it may be that you aren't immediately penalised for using it, but if restrictions do come in - and they will, more slowly than I'd like, but sooner than many think - then the resale value will be hit hard.