Ok,, here are the details.
The Sodium valve was used from 03 to mid 06 on al the 4-tec engines. They are welded together and can fail at the weld. When they do, it causes MAJOR damage. It can cost 4 grand to repair it.
In the 155 engines they almost never fail. Only a slight increase if they are used in salt water. With the SC engines that are in salt water they have the highest failure rate. The boats almost never fail no matter where they are used for the mere fact that you can't run the engine(s) like you do in a ski.
It costs about $50 a valve. If you pay someone to do the job it will be right around $400.