There are not enough free squares left in a row or column. The row or column can never have the necessary number of ship segments. This rule is encountered as the last contradictory step of an indirect logical sequence.
In the above board, square (D,4) cannot be a ship segment. Making (D,4) a ship segment would make squares (E,3-5) water, leaving only four squares in row E as possible ship segments. Since row E has a tally of five, there are not enough free squares left in the row. Therefore, (D,4) must be water.