Long ago the impious mariner sailed from war, trying to return home with the gods always thwarting him (think Odysseus).  After so many years he yearns for the adventure and women of that voyage and abandons the family that he worked so hard to return to.  His old crew being mostly dead he hires a new one.  Many are nervous about joining the blasphemers crew, so one night many find themselves shang-haied (the PCs can join willingly or be shang-haied).

Once out at sea many adventures take place without much trouble.  Insert mini-adventures here.  Examples:
-The crew is trapped on an island of seductive beautiful women.  It takes one of the female PCs to convince them to leave.
-A sea serpent attacks.
-The ship sails beyond the horizon and the sun sets and rises in the east.
-A ghost ship is encountered.

After a few of these adventures bad things start to happen.  Most take these to be signs that the gods are still ill-pleased with the captain.  Insert bad omens here.  Examples:
-A spider is found on the ship.  This is supposed to signify a blessing from the gods.  The person that woke with it on his face was arachnopobic though, and he smashed the spider.  The gods do not take rejection lightly.
(these next two don’t apply to the classic Greek ship, but you can use a tall ship)
-When the crew awakens some of the long boats are missing.
-The lookout falls from the eagle’s nest.

These bad omens have all been created through human artifice.  The helmsmen (or other crewman) tries to start a mutiny.  First subtley, but he starts getting support.  The PCs can figure this out and can react in a few ways: join the mutiny, be loyalists, start their own mutiny.

Here are some ideas the I had for the helmsmen’s motivation:
-Greed
-He could be a kinsman of the captain’s wife.  He would be trying to defeat him in order to give his kiswoman some honor.
-He could be the captain’s wife in disguise.  Trying to get back at the captian for abandoning her.
-He could be one of the shang-haied crewman and be really pissed off about it.