What is the best material to be used when comes to building front seats, rear bench and side panels, marine plywood compared to regular plywood, composite panels like starboard?

May not be the best idea, but I used regular exterior plywood and then wrapped in fiberglass and epoxy to waterproof on the pieces that would be constantly exposed to water.

Buddies boats are just treated plywood

Go to the bog box store and look for some MDO plywood. I got some at my local Menards. Its a resin impregnated plywood made for outdoor use that's used for road signs etc....i rebuilt my interior with the stuff...cuts and acts just like any normal plywood but its stiffer and has a nice finish. You may need a heavy duty staple gun though...i used an air stapler.

Use what Wally said , or Ive used 1/8 ply to bend the seat backs . Then after the pieces were roughed in , I put 2 layers of 1.5 CSM on them to hold the
shape and rigidity.

Got MDO plywood today from reputable lumber shop here in Brooklyn, that thing is not impregnated with nothing beside that is nice and straight with both sides smooth and ready for paint, I poured little bit of water on the cut ends and it sucks it in like sponge.
This thing wouldn't last two rains IMO unless is sealed.
As well it will have to be reinforced with glass in order to hold body's in.

Marine ply with epoxy and glass

Not to mention since it has literaly paper on both sides so in order for resin to adhere, that layer of paper will have to be grind off, great.

Well then i'm not sure if they have different grades of the MDO then or not, but the stuff i get from my local place is great...i have a few scraps siting outside behind my garage and they are facing south west so they get all the weather hitting them straight on.....been sitting there for the past year exposed to everything out there and it still looks solid and straight with no warping or rotting on it as i can see yet. Dont know what to tell ya.

have used, http://coosacomposites.com/
very easy to work with