Swift certainly had sailed before, probably many times. An easy example is from Great Britain to Ireland and back.
Swift was born in Dublin, Ireland in 1667. In 1688, he had to leave for Moor Park in England due to political troubles in Ireland. He made many visits to Ireland while living in England, mostly due to his health. He finally retired to Scotland again in 1700.
As this picture so helpfully shows, it's impossible to get from Great Britain to Ireland without the aid of a plane, jet ski or other flying or water transportation.
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