Natsuno doesn't bother responding to the first sentence. If that's where Number 6 is drawing the line after everything, there's no point in trying to convince him otherwise. He'll learn the hard way.

The second part, though...

"I don't know," he admits with a frown, genuinely frustrated. "It's impossible to figure out what's going on in the captain's twisted head. Apparently he 'just like cruise ships.'"

Sorry, he got nothing, their captor is a childish lazy hack.