By the time Byleth makes it to the finish, Dedue has certainly won and is, in fact, sitting on the edge of the pool, watching him come in. His feet dip into the pool casually, as though he hadn't just horribly... sort-of cheated at their race.

"Indeed. It would appear that I have won."

Byleth didn't stipulate any rules to their race, like non-interference. He only has himself to blame for losing.

"By our agreement, you owe a round of drinks."