You gotz a big job ahead of you.

Curious to know why people dont use marine plywood.
Pressure treated and standard are completely different.

To cut most of it a circular saw, set to the right depth (about half
inch)
and a good dust mask to prevent some years being taken off your
life is where i'd start. Taking measurements is really where you
should start off with, from there you can go to town on the demo work.