Locate ships in the grid such that each “lighthouse” (clue with a given number) indicates the number of ships in the same row or column as that lighthouse (it “sees through” other lighthouses and other ships). Each ship occupies a single cell and does not touch other ships or lighthouses, not even diagonally.
(Rules and example from PGP IB)
History of the puzzle
Probably first appeared on WPC 1993. The author of the puzzle is unknown.