It is a small island, with many purple spikes and shards sticking out of it.
Role in series
SpongeBob is trying to "live like Larry," something Patrick regretted attempting. He plans to catapult himself into the lagoon, not knowing the angle of trajectory that would land them in the reef. Larry tries to save them by grappling on to the arrow with a lifesaver, but he comes too late.
They all ended up in the hospital, though it is later revealed that SpongeBob wanted to land there.