Dead Reckoning

Clusters of four or more pay, then tumble. Every symbol that clears drags the trawler a fathom deeper, and at each depth mark harpoon crates drop in as wilds: empty, then with a harpoon that clears a line, then with a net that lifts every symbol of one kind. Hit the abyss and the kraken surfaces, a 2×2 wild tentacle sliding down the board.