Gap Too Small

Medium Used by Smart Hint

No ship still in the fleet can reach these cells in any direction — the gaps around them are too small, so they're water.

Water
Cells that establish the pattern
Where the conclusion lands

How it works

Suppose every submarine is already placed, so the smallest ship still to find is 2 cells long.
The corner cell is walled in by water on both sides — no remaining ship can reach it horizontally or vertically.
A cell no remaining ship can cover is water.