For sure in salt water, it will extend engine life by a significant amount because the cyl heads will not be likely to rust through after 10-15 years of use. Usually the blocks don't at least I've never heard of anyone having a block rot through here but I have heard of a few cyl heads rotting through behind the valve seats. When I took mine apart the cooling passages of the cyl heads were getting eroded and no telling how thin the cast iron was behind the valve seats.
Freshwater, well if I have the choice of winterizing by doing it the way I always have (a pain, take out rear seats to get out the dang block drains) or, simply hooking up a pressurized winterizing tank and running the motor, & fogging it, I'd say it's worth every penny.
If you can use a full system, a pair of Barr manifolds which I consider good aftermarket is about $600. I have to replace the whole system usually every 7 years, so less time to winterize it every year, only having to replace the elbows (about $430 the pair), longer engine life and better resale would make me do it personally.
My engine in my 21 years of salt water use is on it's 4th exhaust system! The OEM batwings, 2 replacement batwings, and the OMC/Volvo 2 piece conversion.
BTW, GM designed these engines to run with a pressurized cooling system with antifreeze. They will last longer that way. Even in fresh water you can get sand, silt and crud in the bottom of the block passages which can clog the block drains.