For all his skills and talents, Byleth simply was not expecting this. He gives a garbled squawk when Dedue grabs him, wriggling and squirming in his grasp while he's toted along. He really is completely powerless in this case. Given that his powers no longer exist here he only has the strength of his natural build, and compared to Dedue? Not happening.

"Dedue?!" He manages as he's hefted up, just in time to be tossed like a sack of potatoes back towards the other end of the pool. Have you ever heard the Professor shriek?

Now you have.

He lands with a splash, surfacing seconds later, sputtering and spitting out water. By the time he manages his way back, Dedue has long since won.

"Seriously!"