What's a nice material to make a transom plate for a boat that's used on a river or stream?

Prolly not.

Wood Plastic Composites are durable, recyclable, resistant to rot and insect damage, saves on heating costs, has outstanding resistance against moisture, can be painted or nailed just like pure wood and friendly to the environment and therefore an excellent alternative to tropical hardwood. This WPC can be used widely for decking and Wall cladding......

? 

We make ours out of aluminum around here.  Mostly for hydro-streams and STV's.

( too bad you can't hear those Mercury  bridgeports screaming over the wind)

dahmmmm  ,    you could surf that rooster tail   ;-)

I wish I could say these were mine, but it's a video i know of them on YT.

I'm hull hunting for either an old STV or Hydro-stream V-king to get up and going.  I have a few friends who's do in the 95-115mph range in the right conditions and motor jacking.

The rooster tail on these is because the shallow water intake lets them get the prop real high and really only half the prop is in the water.  They motors  are surface driving in reality.

i prefer 2 wheels  ..  don't do  water  much .. (  bad sea cadet  flashbacks  )   ;-)

I like anything that burns petrochemicals and hauls a$$