Well seeing how Mr. Marston is an experienced sailor that has problems with the eldritch-b!tches; I think we should stick with treasure hunting first that leads us into monster hunting. Perhaps the gold we plundered from a temple or ruins was actually cursed gold that woke up some terrible creature that births horrible monstrosities from its slumber, and the purpose then would be finding it, getting a way to go into "the abyss" level of the ocean, and then figuring out how to kill it with something other then brute strength.