The advantage of doing this is that as long as Mark doesn't let go of the tree, Arnold, who is tightly protected in his arms, will never again be washed away by the sea due to failing to grab the tree.
This is precisely the reason why Mark did this.
Arnold suddenly found himself in Mark's arms, feeling a bit uneasy, as a touch of redness involuntarily appeared on his pale face.
At that moment, Mark's voice sounded again above his head, "Arnold, you've got quite a slender waist!"
Mark had wanted to say this for a while.
Now with Arnold firmly trapped between himself and the tree, Mark couldn't help but blurt it out, comparing his own broader waist to Arnold's petite one.
Because of Mark's words, Arnold stiffened involuntarily.
