Any person who—
(a) wilfully and unlawfully removes or disturbs any fixed object or materials used for securing a bank or wall of the sea, or of a river, canal, aqueduct, reservoir, or inland water, or for securing any work which appertains to a port, harbour, dock, canal, aqueduct, reservoir, or inland water, or which is used for purposes of navigation or lading or unlading goods; or
(b) unlawfully does any act with intent to obstruct the carrying on, completion, or maintenance, of the navigation of a navigable river or canal, and thereby obstructs such carrying on, completion, or maintenance;is guilty of a crime, and is liable to imprisonment for 7 years.