The dryad is not really alive as a ghost, but her will has merged with the wood of the ship -- everything, not just the keel. The ship itself is now a sentient, cursed *thing*. The wood (and the dryad) are dead, all that remains are the malevolent will.
The primary effect of the curse is that the wood of the ship wants to return to land. The most direct way to do that is to run the thing aground, or if that fails, to smash it to flinders on some reef and the resulting driftwood will eventually find its way to land.
You could treat the ship as an intelligent Ego Sword. Or you could give it some magical effect like Dominate. It should be sneaky and subtle. I'd suggest...
(1) The ship can "influence" anyone who touches its wood directly. Just laying a hand on the ship can put you under its spell. Those who are so enchanted acquire a subconscious imperative to run the ship aground or destroy it. The longer they remain aboard, the stronger the spell becomes.
(2) Those who sleep aboard the ship have nightmares, a feeling of being in a dark misty forest with enemies prowling around, and being utterly unable to escape. Or dreams of being drowned. Or dreams of being killed by an axe.
Eventually the enchanted sailors succeed in sinking the ship on some reef. By this time the magic is so strong that instead of being destroyed, the ship becomes a ghost ship able to become ethereal. The enchanted sailors become zombie like creatures with a dryad like ability to merge with the wood of the vessel and hide within. The ship now begins to take its full revenge on all who sail the seas on wooden vessels. A common tactic is to appear as an abandoned ship, wait until someone boards, and then the ghost-sailors emerge and slay the boarders. Another tactics is for the ghosts to pose as normal sailors and accept passengers or cargo at a port, which is never seen again.
A clever magician might find a way to use the ship to travel between planes, and a necromancer might be able to somehow control it for nefarious purposes. The ship might sometimes appear in naval battles to help the smaller fleet, in order to ensure the destruction of as many vessels as possible.
The only way to lay the ship to rest is to somehow dismantle it and return the wood to a forest. This could of course cause a new problem unless some means is also found to put the spirits of the sailors to rest... Dismantling will be difficult as the entire ship is an animated object, magically difficult to damage (fireproof?), and can phase into the ether at will.
Mike
