Need to replace the bulkhead wall on my '91 Sailfish

[reelrelief]

I have '91 Sailfish 25, and the top half of the bulkhead wall has become soft. It doesn't hold wood screws well anymore.
This is the portion of the wall that is above the deck, to the starboard side of the door, supporting the helm.
This doesn't appear to be a huge job, but most of my project turn out to be much more than planned. Looking for someone who's done this, for advice. And, looking for advice on replacement material. The original appears to be 1/2" marine plywood with formica on both sides?
I might also have to do the side wall of the head, as it's becoming soft at the bottom. I'm assuming this is the same material?

 

[Shutterbug]

Replaced both sides several years ago. Mine started rotting at the bottom. Strange to me that yours is at the top.

Here's what I did.
1. Removed existing
2. Bought 3/4" marine grade plywood (don't think I needed to, think exterior would be fine) and formica as near the color of the boat I could find.
3. Used old as a pattern and cut with a good jig saw. Fitting the plywood took some time having to make some additional small cuts.
4. Sealed the ends of plywood
5. Glued formica on both sides and cut with a rotary tool (dremel, etc.)

 

[BobP]

The bottom of the head side wall is where water collects in rear floor of head and Grady didn't seal (properly)the panel where it meets the floor.

For this panel that screws off, you can repair the rotted area by making a 3 inch wide bevel and bond together a new piece of 1/2 ply with thickened resin. You can reuse the formica.
Go back (up) beyond the rotted area say 1.5 inches on inside of panel, and set the depth to 1/8 inch on the circular saw and cut the formica only, pry off formica carefully not to break it. Now you will have formica staying on outside. Go up till you reach good wood, draw a line further up by 3 inches, now make the 3 inch bevel and snad off teh remaining rotted wood from the formica. Get a scrap piece of 1/2 ply to make up the size lost, overcut it bigger, make matching bevel, doesn't have to be perfect, now bond the bevel and the rest of the new wood to the formica that is there with thickened resin set on a flat table and clamp it, when set, belt sand smooth, then bond back on the formica on the inside. You may see a slight line on the inside of head wall, but no big deal. Cut or belt sand the right perimeter dimension of overcut new piece of 1/2 inch, and triple coat edges in resin. Caulk in good when screwing back in.
It will never rot again.

On my boat I had to take a angle cut - seemed like a 45 deg angle of rot getting higher towards the rear.

If you are doing the cabin bulhead too as suggested by other member, it may be easier to start with a fresh piece of wood since you are getting new formica anyway. You can use so called project plywood at the depot, or furniture plywood (birch plywood multi layer) as these are not treated - check for warp select wood that is flat.

I'd not go with treated plywood for such a project, untreated only.
And make sure you triple coat end grains so it is smooth like glass.