I read the regs last night and I note that the rules propose 2010, small manfs have til 2011 and >500bhp have until 2013. Plus the actual emissions standards for very large engine still seem to be in the form of discussion type documents with questions being asked of the manufacturers rather than the prescriptive "you will do this" sort of thing.
@Dmoore, If you read the regs and discussion documents, the authorities are already working with the manufacturers on what is possible including testing catalysts in actual boats. The sensors would be upstream of the cats anyway, so the cat would keep them dry. Keeping water reversion low is certainly a challenge but cats significantly reduce the amount of pulsation downstream anyway so that would be less of an issue than with an open exhaust.
It's not simple, nothing in engineering ever is but it is interesting. There is a large section of the preamble which goes through estimated annual US boating emissions and it's tens of thousands of tons of pollutants. Given that a cat would reduce these by 90% (Marinisers own figures in the discussion documents), I don't think it's very helpful to say that emissions would be unaffected. As engines are replaced and newer boats replace old, the whole "national dock" would become cleaner and emissions would be reduced. SInce you'd also save fuel with a more modern EMS, is that a bad thing?