Im just curious what the best screws are for putting plywood on the benches in my jon boat.  I am planning on using 1/2 in ply and then bolting the seat mounts to that wood

I read your previous post when you bought your jon boat and didn't see any mention as to whether it was aluminum or fiberglass or poly?? IF you have access to get to the underside of the benches in any case, I would not recommend SCREWS! I would use small carriage bolts with a good size washer underneath to help spread out the load that will be placed on the benches by the plywood and the seats you intend to mount to it. Will the boat be used in a fresh water environment only??

I don't care what kind of fasteners you use, but make them stainless.  The previous owners of some of my boats need to be shot, for the trouble it causes me to dismantle something.  

I dont really have access to the underside of the benches.  and it is an aluminum jon boat...so do you have any other suggestions

Here's what I did. I used 3/4 in exterior plywood cut to the same size an my factory benches. I then applied a good dose of construction adhesive between the plywood and the benches. I then laid out a grid like a checkerboard, and set stainless steel screws through the plywood into the factory bench about three inches apart. I then fastened the seat pedistals to the plywood using the fattest screws I could find insuring that they pierced the factory benches too. All holes were pre-drilled pilot holes. This is a 14 ft JohnBoat with a 6hp only. I weigh 200lbs, and the majority of the people I have taken with me out weigh me, and the seats haven't budged in ten years. If I ever put on a larger motor, which I seriously doubt I'll do, I'll take another look at this particular part of my setup.

Thanks man I really appreciate it...I'll be sure to use the adhesive and some large screws

I don't care what kind of fasteners you use, but make them stainless. The previous owners of some of my boats need to be shot, for the trouble it causes me to dismantle something.

Times Two.

Actually, times One-Thousand.

Make sure they are stainless if possible.