A 1-clue is already complete, so every cell sharing an edge with it must be sea.
Before: several 1-clues still have open neighbors that could be land or sea.
After: each 1 is sealed by sea. The sea pieces must join later, but do not guess the route. Check island sizes and nearby 2x2 squares first.
Use island limits to shape the sea
Separate clues with one-cell gapsIf two clues have one open cell between them in the same row or column, that middle cell must be sea.Seal a complete islandWhen an island reaches its number, every open cell that shares an edge with it must be sea.Grow through the only exitIf an island still needs land and only one neighboring cell is allowed to join it, that cell must be land.Break a 2x2 sea blockIf three cells in a 2x2 square are sea, the fourth must be land. Then check which clue can reach it.
Protect both islands and sea
Every land group must contain exactly one clue. Before adding land, also make sure it does not cut the sea into parts that can no longer join.