Mutant Zero RPG: Dead Blue Sea - Rules of Play says:
Dead Blue Sea is the second official supplement to the Mutant: Year Zero RPG. This booklet gives you a whole new way to play the game - at sea...
Dead Blue Sea introduces a whole new Zone at sea, complete with a full-color map, plus information on new mutations, monsters, maritime artifacts, and rules for Zone travel at sea, including navigation, weather, wind, and diving.