Devilspawn — David Sakmyster

Duncan Miles set out to be the greatest merchant trader on the high seas, but destiny had other plans. After a fateful decision to save his life and his ship at the cost of his soul, Duncan pledges himself to the most fearsome pirate on the seas, embarking on a seven-year spree that only ends when he can stomach the captain's cruelty no longer.

The ship, renamed the Devilspawn, sinks after Duncan's eventual mutiny, but death, he realizes, is not the release he expected. Trapped as a spirit between worlds, he is caught in an underwater graveyard where souls wait in terror for the Lord of Bargains, a powerful voodoo Loa who seeks the one treasure that can make him unstoppable.

Duncan must harness his new-found abilities in this new realm before finding a way to stop the Loa -- and potentially save his own soul in the process.