The water around the island isn't extremely deep but is uncharacteristically dark despite ample sunlight during the day. Beneath the surface is mostly craggy rocks, blandly-colored plant life, schooling fish, and dubious looking caves that definitely do Not house a variety of sea monsters. There is also an absolute plethora of shipwrecks. (In fact, we may throw a plot discovery from one of them at Delta down the line!)

Additionally, there is a clear division in the water. Past a certain point around the island's perimeter, the water is visibly lighter. Looking at it resembles standing at the edge of a rain storm and seeing the threshold where the rain stops and the sun begins. If Delta attempts to cross this line, he will be blown backwards by a current so aggressive that even the weight of his diving suit will not be enough to keep him on balance or attacked by a massive leviathan. It is physically impossible to exit the darkened perimeter.