From my own sailing experience, you need sail, spellcraft, the more the better. Few sailling enemies will knockdown you. But most of them sneakattacks. Another threat is meteor swarms from some caster enemies. You need the ability to cast elementals shield, which grant you 50% immunity to fire.
Fathomless pact's sail bonus is neat, especially when you are trying to get 100 sail. But party sailing doesn't welcome tentecles, since you will be the main threat to all sailor, and most sailors would die within 1 round or two. The demon pact is also an unpredictable threat to other sailor.
A popular multiclass option among pirates is Loremaster. When you have a steable bond with other pirates, -yoink others aboard is extremely convenient. Sound lance and circle of doom, withering ray, or eldrich doom are welcomed since they are party friendly AOE. Sing sea shanty is good, but bards always sings better.
In general, warlock pirates would focus on their sailing ability as well as teamwork abilities, at a cost of their solo potential or PVP strength.