Notes:

Female Byleth: I'll give you some advice, why don't you try to learn to swim yourself?

Edelgard: Great idea sensei, I will try to learn to swim by myself

Edelgard can't swim in the water, but Edelgard needs to learn to swim, and she went to the lagoon, and she is going to learn to swim by herself. Don't panic, just try it.