The iconography and depiction of the wading figures and the arms rising from the tide is very good, but I would've left out the skull face in the background. The imagery in the foreground is very realistic, the background in comparison is almost cartoony. I would say that it would work better with a kind of ethereal fog glow in the background. If the edges of the background were softer and less clear, that would work as well. Things that are seen can scare (like your slaves/zombies), but things that are half-seen can also be terrifying.
Wow, very, very nice! I love the mists, the fact that you can't see everything clearly just adds to the effect! Really find myself intrigued by this piece...and somewhat terrified actually, although being afraid of a painting might seem a bit silly, but you really managed to give it a scary atmosphere!