If you have a very thin, one-tile strip of land with huge lumps of land on either side, build a town on it so your ships don't have to go all the way around.
A Panama or Suez canal.
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The AI always (in my experience) moves a settler companied with another (fighting) unit.
The settler will sometimes wait for that unit in the early game. It's an easy kill and criplles the AI big time at the start of a game.