Senku looks a little surprised by the confirmation. Or maybe by the straightforwardness of it.

"Damn. You agreed to take a raft even though you knew you'd be screwed if we capsized?"

He's a little impressed. A lot of people wouldn't do something like that. Senku will put himself in danger for science (and for others — he won't easily admit that second part), but this was a trip to save a girl. They could have got other people who could definitely swim, but Levi never even acted like he was going to pass the rescue mission off to someone else.