I think the easiest way to make it clearer is to highlight it in red as a mistake if done incorrectly, as you do with the counts on the row ends. If you try to

I think the easiest way to make it clearer is to highlight it in red as a mistake if done incorrectly, as you do with the counts on the row ends. If you try to place a ship token close to another one then you should also highlight it in red.