From the tracks of the sea creatures, it's obvious that they came inland using the river that flowed to the sea.
It seemed that even though the sea creatures could walk through the land, they still needed to have water every 2 hours.
He and Layton had been observing the sea creatures for 5 hours already, and they saw that those creatures would always go near the water when it was already 2 hours, and then they would soak themselves for at least 10 minutes before continuing what they had been doing.
When they were on land, they still continued what they were doing since they started observing them.
Those sea creatures were all eating and destroying all the plants and trees that were still on the island.
They also didn't leave behind the small animals that they had encountered. If they were only eating all the things that they could see, they shouldn't have seen a lot of only barren land.
