Wow, McCree doesn't even have to offer the guy anything to get him to do this? Nice.

He chuckles and jerks a thumb behind him at the water.

"Not too deep. I can see where I need to go: there are these tunnels down there. You can see the darker patches that must be their entrances, but I can't hold my breath long enough to see what's through 'em. Seems like you might, though. If you're up for it."