No 2×2 Sea

Easy Used by Smart Hint

Three sea cells already forming an L would complete a 2×2 block — the last cell must be land.

Island (land)
Sea
Cells that establish the pattern
Where the conclusion lands

How it works

Three cells of this 2×2 block are already sea, forming an L.
A fourth sea cell here would complete an illegal 2×2 block — it must be land instead.