Is it possible to do a back deck rodeo roll in an 18ft kayak?
back deck, yes
But your head/body will be under water. As I understand it, a true Rodeo roll is a whitewater thing where your head stays out of the water the entire roll.
If you play in the surf, learning a back deck roll can be helpful for those times when a wave knocks you back and the force makes it hard to tuck forward into a standard setup position.
-Tom