The Serpent's Lair is one of the two small caves that the player can find in the floor of the first level of Deep Hole. This small cavern is only home to one sea creature, a Ribbon Moray, and it is also home to the unique, one-time salvage item the Trident Ring.
No other fish can be found inside the Lair itself, but some smaller fish tend to gather near the entrance, like several of the resident Pineconefish the player can find beyond the Mouth of Truth.
The other small cave similar to the Serpent's Lair is the Giant's Grotto, which is home to one of the two resident Giant Groupers that make their home in Deep Hole. There is another cave known as the Hidden Cavern, but that cave is located in Deep Dome (the second and deepest layer of Deep Hole), and requires that the player have a dolphin partner with them to move a pile of rubble out of the way in order to unlock it.