I woudnt use marine grade plywood, way to heavy and seams to be rather wet and tends to require lots of extra bracing. Not to mension, for the cost of marine grade wood you can get aluminum.

Option 1: regular plywood, If you seal your regular plywood with thompsons water seal it will last a life time.

Aluminum sheet. Aluminum sheets are easy to cut, they are light and can be glued to or painted and will never ever rot.

I use aluminum tubing for the structure, after the extension or deck is built i use aluminium welding rods from harborfreight with map gas and lock it all in resulting in a rigid base. The other perk of aluminum tubing is that it can be easily bent with a bender alowing you to really customize your build.

Sea deck is a nice way to top it all off. It looks good and so far pretty rugged.