Use sheets of 4'x8' 3/4" plywood as top hung sliding doors on shed?

Any exterior grade will be fine seal the edges before painting will give the best long life results. The only difference in standard exterior and marine grade is the quality of veneer that is used. Marine grade can have only a limited number of knots and they have to be tight and smaller than a dime. I used to work in a plywood mill and that was the only difference I remember they used the same glue, pre press and hot press process . So you are paying quite a bit for superior grades of veneer if you go to marine grade.

When I built a shed earlier this summer (located in central Indiana), we purchased and had all the materials delivered. One sheet of pressure-treated 3/4" plywood was more suitable for boat-building than the flooring it was intended for. i.e. it was well and truly warped - sitting flat on the garage floor, each end lifted 3-4" off the floor. (Hence my emphasis on "had delivered" - it's a piece that would have never come home with me if I were picking my own lumber.)

I don't recall off the top of my head if this was ACX or not, but plywood, even pressure-treated, can warp if left to its own devices. I wouldn't count on your doors staying flat without some sort of bracing or reinforcement. Yours will be free hanging with nothing to resist any warping forces and exposed to the elements on one side - mine was somewhere in the middle of a stack of plywood (not on the top/bottom as evidenced by the lack of damage from the banding), presumably kept basically flat by the weight above it.

Adding a "frame" of PT 2x4 on the outside in a "barn-door" type pattern (frame around the outside, one or more diagonals inside the frame) would go a long way toward keeping the plywood flat while adding some visual appeal. Worst case scenario - it would be totally unneeded structurally but would still dress up the otherwise drab slab of a door made from a sheet of plywood.