Two question about migration:
1. Can I transport citizens by engaged ships?
2. Can I transport citizens between 3 or more regions by using two ships (staying on departure and arrival region)?
1. I don't think so (no rules at hand), since you need to be able (at least theoretically) to place the ships on the connections between the tiles. This is not possible for engaged ships (that are fishing, for example).
2. Yes, you need 1 ship per "link", i.e. tile border.