"Marine" gaskets are basically if you are running in the ocean, ie SALT water, for freshwater you can spend the extra money, but is not necessary, just my .02
WRONG!
Marine head gaskets are not just for salt use....Marine engines do not have the advantage of the anti corrosion protection of anti freeze. Thus making the electrolytic effects of brackish waters left in an engine most detrimental to the survival of the sealing surfaces...
I use brass freeze plugs and marine head gaskets in all boat engines. Cheap insurance IMHO...
A trait that marine gaskets also have is dielectric bonding of the sealing surfaces, this is like a good primer on bare metal on a paint job, it keeps the free iron from being exposed to the gaskets metal, which is how they function to begin with.
This has been a major debate among the boating community for decades, it is my opinion that the only time "standard" gaskets should be used is when the engine is not expected to be together for more than 6 months (read; full race deal)...
Sorry for the harsh intro, I feel strongly about this because I have taken apart literally dozens of engines that could not be salvaged because of the amount of corrosion in the sealing surfaces of the heads and block...

GT