If you will want to use the distance expression.
If you want the player to follow the AI's, you will make an event that will activate another once the player is X distance from the AI ship it must follow. The activated event will become true once the player is X plus something distance from the AI ship you want to follow. Then you can link it to a goal making the goal false. You can use warning for the player in other events that were activated by the first by putting a distance expression but with a lower distance to the AI ship you want the player to follow and then send a message in that event.
If you want to have just the player following nav bouys, you just have to put a distance expression in the bouys you want him to follow that's all there is to it.