Tap the water to send a ping. The four numbers show how many empty squares the echo crossed each way before it hit a stone. An underlined number means it reached the edge and found no stone. Switch to Mark (or press and hold a square) and flag the squares where the stones must be. The puzzle ends when the flags are right and the pings prove it — when “possible” shows 1. A lucky guess does not count. Fewer pings is better. Finish the first pool to unlock the second one, with three stones.
Keys: M switches Ping and Mark, Z takes back a flag.