Mega Tile 15

Publisher's blurb:

Mega Tiles are just what the name implies: really big geomorphs compatible with Dungeons in Blue tile sets. Each Mega Tile is a single geomorph covering four times the area of a standard-sized tile. These larger tiles allow bigger caves and chambers to be depicted, and also serve to break up the gridded feeling geomorph-based dungeons can sometimes have.

Mega Tile Fifteen is a watery cavern complex with a few signs of occupation: a magical seal, a small dock, and a few boards that provide access to a rock out in the water.