The Sea Mystery is a 1928 detective novel by Freeman Wills Crofts.

[1] It is the fourth in a series of novels featuring Inspector French of Scotland Yard.

[2] As with a number of his works Crofts creates a puzzling mystery which French is then able to solve using a Tide table and Bradshaw's Guide to the railways.

[3] The plot has some similarities with his debut novel The Cask (1920).

[4] French of Scotland Yard is called in when a fisherman discovers a crate containing a battered body on the cost of South Wales.