Ghosts fly around in the fog, trying to disorientate your ship. You have to pay attention and shoot them when you see them or they will grab your wheel and turn it, drop your anchor, turn your sails, and raise your sails.
This would make fog more interesting as its intention is to make things spookier, so a battle in the fog would be quite difficult and quite spooky. It works too because a larger ship has more players to manage this and a smaller ship has all of these things closer so a solo player could also manage dealing with ghosts in the fog.