A lot depends on what "stainless" is used. The definition of stainless steel is a mix that that contains at least 10.5% chromium. The typical ferritic stainless steel (405, 409) used in exhaust systems, mufflers, etc. has the lowest chromium content at 10.5 to 12%. When you buy a low dollar "stainless" muffler you're getting a muffler made out of the cheapest stainelss steel. Those typically last about 25% longer in use then galvanized (aluminized) steel.

More expensive stainless mufflers (Borla in particular) use austentic stainless steels (304) which have a 18 to 20% chromium content and these will last much, much longer then any coated steel. This is why there's a big price difference between shops when a customer is quoted a price for a stainless steel exhaust. In my case, I only quote for 304 (or 316, 321, or 341) materials.

Thanks, Mike and John! Yeah, marine environments are brutal! Nothing would surprise me. We're fortunate to live in a fairly protected area (good for our cars), but of course our boats go out on the bay. And yes, cheap SS does corrode. At least now I know more about the grades of SS, so I can be a smarter consumer.