Originally Posted by tomgoodman

"The cat's out of the bag" --- it had nothing to do with felines, but referred to the "cat-o'-nine-tails" and the silken bag in which it was kept on the old sailing ships. The Captain might occasionally threaten a flogging, but if he took the cat out of the bag --

"Scuttlebutt" --- the barrel of drinking water, around which the crew swapped rumors.

"The devil to pay" --- the outermost deck plank (the 'devil board') needs a coating ('paying') of hot pitch, and the hapless sailor has let it get cold. Obviously, it would be hazardous to find oneself "between the 'devil' and the deep blue sea".

Source: the excellent series of novels by Patrick O'Brian, starting with "Master and Commander".