I'm writing AI for the game and encountered this article that helped me out. I'm not sure how the probability function is computed. Does it rely on some advanced math I'm not understanding or for each move program generates randomly lots of possible set ups and then computes chance by counting times ship is encountered on the given field?
You enumerate every possible legal position that the largest (surviving) ship can be in. Call that N. Then for each cell, you count up how many of those positions include that cell. Call that c. Then your probability is c/N . You can deliberately targeting the largest ship as the probability map is more concentrated for that one, and therefore most likely to give a successful hint, although the authors continue to do the same for other ship sizes.
Whether this is a good measure is debatable - it assumes all remaining positions are equally likely, which in turn assumes your opponent scatters his ships at random. I suspect humans will tend to follow patterns they believe makes life difficult for the opponent, eg. not having touching ships (so that hitting one won't lead you into hitting another while in TARGET mode).