Let's say it's a 10x10 peg board, and you are choosing one location each try. Some number of those locations are success and some are failure, depending on the number and size of ships on the board. So the first try is x/100 to succeed, where x is the number of pegs containing enemy ships.

The second try is different in several ways. First, there are now 99 possible choices. Second the selection isn't strictly random, because you now have information on whether the first try was a success or failure. And third, the number of possible successes depends on whether the first try was a success or failure.