Once you were on a ship, you were expected to stay with that crew and loyalty was prized. Of course, it was also forced and that is how we will eventually get to the real reason why pirates couldn't swim. Unless you lived on the ocean or by a lake, you never really had any opportunity nor inclination to learn to swim.

Is it true that most sailors cant swim?

Most sailors, up until the late nineteenth century could not swim. ... This popular tale did not, however, inspire many real life sailors of the eighteenth century to learn how to keep themselves afloat.

Did all pirates know how do you swim?

There is certainly evidence that some of the sailors during this time couldn't swim. ... However, others could swim, although not very well. When one of pirate captain Edward Low's ships overturned during a careening, captured fisherman Philip Ashton wound up in the water.