He, Ron, and Hermione left the castle together on Saturday and set off through the cold, wet grounds toward the gates. As they passed the Durmstrang ship moored in the lake, they saw Viktor Krum emerge onto the deck, dressed in nothing but swimming trunks. He was very skinny indeed, but apparently a lot tougher than he looked, because he climbed up onto the side of the ship, stretched out his arms, and dived, right into the lake.
“He’s mad!” said Harry, staring at Krum’s dark head as it bobbed out
into the middle of the lake. “It must be freezing, it’s January!”
The word ‘bob’ means to move or cause to move up and down repeatedly, as while floating in water. But I don’t understand the phrase “bob out into”. It doesn’t seem to make much sense to me. How should we understand the phrase here?
If the sentence had been “…dark head as it bobbed in the middle of the lake”, I’m sure you would get the image that Krum was already in the middle of the lake, floating there, with his head bobbing up and down.
But since he’d only just dived in, it is described as “…dark head as it bobbed out into the middle…”. He’s swimming towards the middle of the lake, using a style something like breaststroke, where all you’d see would be his head bobbing up and down as he progressed.
I agree that if you’re imagining him swimming freestyle (arm over shoulder), then “bobbing” would not be a good description.