"Sailing under a false flag"
This refers to deceptive maneuvers or covert operations conducted by another, third party to conceal identity The action is thus actively attributed to an uninvolved third party for appearances. The actual actor is thereby acting "under a false flag." In English, the much-publicized deceptive maneuver is also called "sailing under false colors," while a courageous flagger is sailing with true colors.
"Being washed with all waters"
The expression is used when someone has cheated his way out of an unpleasant situation. Sailors visit many countries and get to know different cultures. If a sailor has been around a lot, then he has been washed with all the waters. With all the waters of the seven seas.
"To keelhaul someone"
In its more common meaning, keelhauling - also 'kielen' - refers to a severe disciplinary punishment common in seafaring until the 19th century. The punished person was pulled through under the hull of the ship with a rope. The person being punished was pulled along under the ship's keel at sea either athwartships (i.e. from one nock of the main yard to the other) or fore and aft (i.e. from the bow to the stern). Keelhauling was often fatal given the severe injuries caused by rough debris such as sharp-shelled barnacles on the ship's hull. Decisive factors included how fast the rope was pulled and whether the punished person could swim or dive to keep himself in sufficient distance to the hull.
"If someone would be found sleeping on his watch, he shall be stolen 3 times." - Johann Christian Lünig: Corpus iuris militaris, 1723
In 17th-, 18th-, and 19th-century seafaring, keelhauling was considered, along with running the gauntlet, the most severe of corporal punishments.
"Clear the ship"
To "clear the decks" is to tidy up and clean thoroughly.
Sailors scrubbing the deck on board HMS RODNEY, 1940
"Battening down the hatches"
To batten down a bulkhead means to close off a part of the hull where there is a large leak so that the rest of the ship will not also be flooded and the ship would sink. The ship and crew may be saved in this way.
In addition, bulkheads stiffen a ship, both transversely and longitudinally. Those who colloquially "batten down the hatches" not infrequently stiffen themselves, namely to a point of view sacred to them, and are thus no longer open to dissenting opinions.
"Showing the flag"
This saying is related to showing one's colors. Ships had to show to which nation they belonged. In the past, there was no radio, so you could only roughly estimate from a distance through a telescope whether friend or foe was approaching. Sailing under a false flag was just the opposite: dishonestly hiding one's true motivations. Pirates, for example, using a stolen flag or ship, pretend to be harmless in order to get close enough to their victims and capture a coveted ship.
"Now that's a lightweight sailor"
Imagine an ordinary seaman lying down on a deck chair, daydreaming and forgetting about the work he actually has to do. This saying is neither nice nor historically correct.
Real light sailors, who officially existed until thirty years ago, had their hands full in their third year of apprenticeship as a seaman and no time at all to lie down on a deck chair during that time. Besides, a seaman worked in the merchant navy, not on passenger ships with sun decks. The ordinary seaman ranked above the deck boy (1st year) and the young man (2nd year), but below the seaman with a letter, who had completed his three-year apprenticeship as a full seaman.