Re: Kind of wood for Side Panels, bench seat??
You don't need marine ply for those appliations. Exterior ply, protected w/paint or sealed w/epoxy (or both) will do fine. What ever you use, I recommend protecting both sides as much as possible. I had a dry rot problem with a seat and some hatch covers in my aluminum skiff as a result of the underside not being protected.
My 1982 boat has MDO (one side) ply bench seats. With the exception of the one that was nailed by dry rot they are still going strong after 30 years - although now in need of a coat of paint. When I repaint, I will remove the seats and seal the undersides and edges with a penetrating epoxy and then paint both sides before re-installing. I expect that treatment will allow the seats to outlast both the aluminum hull and me by a wide margin.