i don't think it was that they last longer - the design is made so that if there failure of the exhaust components, the water stays out of the exhaust port and keeps water from back-flowing into the engine. At Least for a while. By running the water through the separate ports/channels in the dry joint - you should notice an increase in engine temp due to the flow restriction before you put your engine at risk. Where as with the "wet joint" by the time you notice the issue with an increase in engine temp, it may already be too late.
So the repair/replacement time in salt water should be the same. But failure isn't as "catastrophic" as with standard manifolds and risers.
