Algae grows fast in still water. The water here isn't flowing that much. It's mostly still. If the boat was in the water for a few days, the bottom would be covered in a layer of algea which is sticky and slimy. [ Hence, touching it. ] There's not much of a layer on the boat which means it hasn't been in the water for very long.

[ He rolls back onto his heels and stands. ] It doesn't mean someone's here, only that the boat hasn't been here long.

[ He was taught detective work at UA, so he takes what he says and adds it to the image he's forming in his head of their current situation. ]

Hikers use piles of rocks to indicate where they've been. Maybe they're exploring like us.

[ Did they... get lost? He almost frets at the thought and looks over Touya's shoulder. ]

We can check out one of the tunnels. Maybe there's something of interest there that made them take it.