An important note about sailing with Jack Sparrow is that he keeps both an impeccable ship and a ( generally ) clean crew. If a man ( or woman ) should board his ship and he should discover at any point that they cannot swim, chances are fairly high they will be taught at least some rudimentary basics in their time under his tenure.

Everyone aboard the Pearl needs to be able to jump into the water at a moment’s notice to attend to outer damages in the event that the gear on board is not efficient enough to block holes below the waterline. Everyone needs to have enough skills to manage this because simply having three or four is not good enough on a pirating vessel. Battle is inevitable, men dying is an inescapable fact, and Jack will not run the risk of losing the Pearl because the few people capable of sealing her wounds have been taken out in the skirmish.

This is the argument he provides to the reticent. There is, however, another reason for this and that is that Jack honestly does have limits for how ripe he will let his crew get before he demands that they swim and clear themselves off at the very least. Refusal to do so has seen men thrown overboard, so it is generally a good idea to at least take those lessons when they are on offer.  

Jack and his crews tend to swim fairly regularly, and because of this being becalmed is not so terrible as it can be for other vessels – provided their rations are in a good place, at any rate. Though not all crew members are taught by Jack directly, especially as crews tend to overturn regularly, his core crew all have the skill, and when he can be discerning about whom he hires ( which is less often than he would like ) it is generally one of the questions asked prior to hire.