A middle ship segment is flanked by water on two adjacent sides.
By definition, a middle segment always has a water segment and a ship segment on adjacent sides. This rule is encountered as the last contradictory step of an indirect logical sequence.
The above board show the consequences of square (I,5) being a ship segment. Making (I,5) a ship segment turns squares (G,5) and (H,6) into water. This leads to the impossible situation where the middle segment at (G,6) is flanked by water on adjacent sides (exactly one of these sides must be a ship segment).