Talk to Jotham in Fogsbane, the lighthouse near Britain, and he'll tell you about whirlpools that drag ships to the underworld. It seems that someone survives the fall, so let's look for them. If you sail your frigate in a whirlpool, you'll get stuck in the underworld, unless you know the Word of Power to open the corresponding Dungeon. Go to Britain and talk to Annon: he'll tell you the Word of Power to open Dungeon Despise: this is the dungeon where the whirlpool sends the ships.
Explore the underworld of Despise, possibly with a magic carpet, and you'll meet one of the last party members: the mage Captain Johne.