If all submarines on the board can be accounted for, you may be able to finalize other squares as ship segments. In this case, you know that other finalized ship segments cannot be submarines, and therefore must have an adjacent ship segment.
Note that you do not necessarily need to know the exact positions of the submarines. Knowing which rows or columns they reside in may be enough.
The board shown above has four submarines to be found.
Three finalized submarines are shown: (G,6), (G,8) and (I,8).
Examining the board shows that the last submarine must reside somewhere along row I. This is because wherever the remaining ship segment in row I is, it is a submarine.
Having accounted for all four submarines, we know that the ship segments (D,2), (G,2) and (G,4) cannot be submarines. If any of the squares (E,2), (F,2) or (F,4) were water, this would create an unacceptable fifth submarine. Therefore, (E,2), (F,2) and (F,4) must all be ship segments.