"Right...but the fish bikes have an engine. I'd think they'd be more than twice as fast as a human."

Luca's pretty fast himself, having fins and a tail and everything, but that doesn't mean he wants to swim in circles around a giant creature and hope he's fast enough to not become lunch.

"I'm sure the Leviathan would be."

It's large, so it might take some effort to get going, but once it gets moving it seems to be very fast, after all.