You can either purchase a new engine equipped with emissions control, or use an engine family exempted from that requirement.
Check out the link below and click on one of the Marine Power
units. It states that they're not required by current EPA regulations to be catalyst compliant until further notice.
It gets a little more complex if you're repowering a boat. Options and regulations are explained here :
As far as I can tell, vintage engines, used engines and home-built custom engines are exempted from the regulations.
Paul Kane Chelsea, PQ
Building the Glen-L Hot Rod : http://www.boats.chelseacoachworks.com