Landlocked Cell

Medium Used by Smart Hint

A cell fully surrounded by land can't be a lone patch of sea with nowhere to connect — it's land.

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

How it works

This island has wrapped almost all the way around one interior cell — all four of its neighbors are already land.
If that interior cell were sea, it would be a single sea cell with no path to the rest of the sea — never allowed. It must be land too, completing the island.