[spoiler]Unless you want to just walk along the bottom, you need to make a swim check. Will post more later.[/spoiler]As Marlowe slips into the water, he notices that the shallow edge quickly gets deeper as he approaches the opposite side. It is still only barely over his head, though. He sees the holes that the stream enters and exits the chamber from, but no opening along the far wall where the Barghest gestured that they should go to find the aboleth. As he goes to touch the wall there, he finds that his hand passes through it effortlessly, though.
Passing through the illusory wall, he finds that the cavern opens up into a chamber that extends outside of his line of sight. There is an island in the middle of this watery chamber, though. From the island, he can see that the chamber is about 100 feet across and more or less round, with the ceiling going up to about 50 feet above his head. There are three visible exits.