Most ski boats already conform to the 2006 standards, which is the target date for the cutting of emissions. The deal with that is, that they are looking at the recreational boating industry as a whole, and aren't targeting specific boats for cat/con installations (at least nationwide--CA is most assuredly a different story). You are starting to see the marine industry slowly move away from 2 strokes in both outboard engines and PWC's. Four stroke outboards are becoming much more widely accepted, and so are 4 stroke PWC's. By making this shift, the marine idustry is reducing the overall output of florocarbons across the board. CA is by far the most stringent state, and most manufacturers who sell here in the US make their entire US product line 'CA approved'. Also, the remaining 2 stroke applications are moving to direct injection as a means of reducing pollution output. We recently outfitted the family offshore fisher with twin Yamaha 225's that are two-stroke, but fuel injected. Fuel efficiency is supposed to be greatly increased as well. (we test drove an identical hull to our boat w/ the 4-strokes on it, and just didn't feel like we could sacrifice the power, which was quite noticable)
The PWC industry is moving towards four strokes as well, with all manufacturers now having at least one four stroke offering, and Honda, who is only two years into the PWC market offering NOTHING but four strokes.
I own three Yamaha waverunners, and two of them, both with the big 1200cc/155 hp engines came with catalytic converters installed. The replacement cost for these catalytic converters are around $800. There is a $53 part that is sold that replaces the cat/con, gives more power and acceleration, and better throttle repsonse besides being 1/15th of the cost. Cat/cons aren't the answer because of their cost. Cleaner more efficient engines are the way to go, and most boaters being perfomance oriented anyway will revolt against the use of cat/cons in their boats, and most likley just simply remove them like I did mine. No way I was paying nearly $1000 for something that a)decreased power, b)actually hurt fuel economy, and c)was meant to make it comply to CA standards. Most people would view it that way too.