Coral Cave is located north east from monastery on an small island.
To access it you need Coral key, which can be obtained from crabs.
Coral Cave contains crabs (level 2, 6), silk crabs (level 15, 25) and some ores.
Second door requires also a coral key (the key might break when trying to enter). Second area contains a maze with level 25 silk crabs, and roaming massive waves which will knock you back to the entry door.
At the end of the maze there's a door to Pirate cave
Red Crab is in Coral Cave past the pirates, you have to climb up a dark rocky wall at the far back that has a pathway that's easy to miss at first