>>24467798
being able to sail a ship in ASOIAF is presumably not a genetic thing like dragon riding is.
the world of ice and fire says that the in-universe commonly accepted origin of the ironborn is that they descend from first men that sailed to the islands thousands of years ago. maybe the children brought the hammer of water down and then they learned to sail when they were already in westeros. or they always knew how to sail but it just wasn't a big part of first men culture for whatever reason.
there is also another theory posited in the book that they descend from a group of people that came from the west of westeros (clearly retarded).
plus the full title the king has is "King of the Andals, the Rhoynar, and the First Men, Lord of the Seven Kingdoms, and Protector of the Realm" - it acknowledges that he is the ruler of the ironborn and their kingdom but does not acknowledge them as a distinct race. and that's because they're just first men with a unique culture separate from the main one in the north.