Byleth counts down to the start of the race, and Dedue readies himself to spring into action. One might expect that, upon hearing go, he would attempt to swim as quickly as he could-- one would be incorrect. Instead, his hand snaps out and closes around Byleth's ankle, wrangling him back within arm's reach. Then he gets an arm around his waist and hauls him up against his side like he's holding a football. Contained-- one professor.

Dedue wades out into the pool towards their designated finish, Byleth still held securely. If the professor even attempts to struggle, he will find that his flailing is ineffective at best. Dedue has found that the lack of Crests in this world significantly bridges the gap that used to exist in terms of strength; Byleth, like Dimitri, is now but a mere mortal man. Dedue can make it roughly halfway before the water starts to become too deep to comfortably walk in, and at that point he stops, adjusts his grip on Byleth, and throws him like a sack of grain back towards the shallow end of the pool.

With that head start, and Byleth yeeted back towards the starting line, he has a significant advantage in making it to the finish first.