A little trick you can use to get settlers safely across a gap of, say 4 non-coastal sqaures is to use ship chaining. Some consider it an exploit.
Send out one empty galley into the middle of the straights you have to cross. End turn. If it survives, send out a second galley with the settlers / escort units (thet must have movement left) onto the square with the first galley. Wake the units, and load them into the fist galley. This can then be moved onto a safe square.
Oh, and I know is is hasstle to end up on a little island, but remember the AI is likely to have a similar problem. Get those suicide galleys out.