Urchin Grotto

Named after the many sea urchins living in its waters, this grotto is commonly visited by pirates

Used by pirates who need to make a stop on the island, and remain unseen by goblins and gnolls. While it is not permanently occupied, it always has some resources in the scant dry land available for emergencies (which is, usually, when pirates land here).
The next people making an entrance here, however, will be surprised, as sea urchins are no longer the main problem one has to worry about when wading into its waters.
A tidewretch has denned here, and hidden the piece of driftwood it is attached to, hidden in the sea urchin seafloor. Sea vermin have congregated here and the undead dryad tends to them, confident in her ability to use them in order to protect her treasure.
General nuisances in the water, who stalk and prey on anyone once they step on a sea urchin and draw blood:

Bone crab

Jellyfish swarm

Giant isopod

Urchins protecting the deadwood:

Black spot sea urchin

Hunter sea urchin

Ravenous sea urchin swarm

Treasure

Hiding in the dry parts of the cave are provisions (rations, dry firewood, a compass, two tents, ten sleeping bags, one suit of armor each of light, medium, and heavy qualities, and five short swords).