I would say the 'punching arm's' front deltoid do alot of work 'punching' the paddle into the water and withdrawing them for recovery. The trapezius probably got worked because of the punching too. The lats are involved too due to the lifting and lowering of the punching arm.
The pulling arm rear deltoid, lats from the pulling action. The forearm got a lot of workout from flipping the paddle.
The main torso muscled of the abs and lower back will be involved due to the twisting motion.
The legs are probably involved in a small way when you 'kick' the boat.
Anymore academic I don't know already.
