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?