It is steel plating to repair the ship. The book (which has a slightly more serious and less slapstick tone than the film - though the character of Flynn is equally larger-than-life) devotes several chapters to the British pursuit of the
Blücher. She engages in battle with the British Cruiser
Orion and the Destroyer
Bloodhound and emerges triumphant but heavily damaged. The ship hides in the delta to make repairs and it is discovered that large quantities of steel plating are needed to effect these repairs, which must be hauled over land to them.
Can't remember, but I think the film glosses over all this without really explaining it.
(Sorry Admiral, forgot about your question - I'm easily distracted by other things
)