Curse of the Sunken Castle

From the description:

Curse of the Sunken Castle is a Fifth Edition adventure for 3 to 7 characters of 7th to 8th level. The adventure is optimized for five characters with an average party level (APL) of 8, though characters of the 7th level can survive this adventure if they are extremely cautious and rest between encounters.

The characters are asked by a local mage to track down a group of explorers that have gone missing on a recent expedition. This adventure begins on a remote island that makes escaping from it impossible; however, this adventure can easily be transplanted to any coastal region.