First, remember that the name "marine" is very misleading. Marine grade plywood is basically the same as regular plywood...Same wood, same glue...The only difference is that marine grade plywood doesn't have any voids in the inner veneers. Basically, every veneer is bookmatched tight. Although it does make a better product, it doesn't make it any more weather resistant.
Similarly, treated plywood isn't all it is cracked up to be. It is glued with the same glue. The wood is just treated to resist insects and decay. It will de-laminate if left exposed to the elements long-term. It is actually made for concrete contact applications on the interior of structures (behind electric panels, furring strips, etc).
Where specifically on your porch are you planning to use it? Please don't say the floor! Knowing the weather exposure it'll be subject to, and the specific use, people should be able to recommend a finish.