"Maybe...the hard part is finding them when we have a Leviathan on our tails!"

Because right now, Luca knows he's more concerned about keeping away from the Leviathan than anything. Then he gets an idea that might help with both.

"I'm going to look for an underwater cave I can look around in."

That should both give him a safe place to hide, and it seems like a reasonable place he might find crystals.