7 Flags At Sea

1-Skull and Crossbones

The most infamous flag to be found at sea, this flag means that the crew of the ship flying it are pirates and the PCs must choose either to surrender and be robbed, to try to flee, or to do battle and risk being slain outright if they lose the fight.

2-Red Banner 

If a warship or pirate ship raises this blood-red flag, it means the fight is literally to the death and no quarter will be given. If you lose this battle, you and everyone else aboard will be slaughtered and thrown overboard.

3-Yellow Jack 

If a ship is flying this bright yellow flag then whatever you do don't come too close, as the yellow flag means there is contagious disease on board and you could easily get sick if you don't stay well away.

4-Blue and White Swallowtail 

This flag means the vessel has divers down that are exploring some sunken wreck or coral cave, or maybe they are just tourists. Either way it's captain will be furious if you speed past, as you are putting the lives of the divers at risk.

5) White Ensign or equivalent 

The White Ensign (or it's fantasy world equivalent ) is a flag that can only be legally flown upon a warship, and such ships pack a formidable punch in combat. Perhaps the PCs are pirates or the vessel is chasing after a criminal on board or it needs to press men into it's crew because of losses from disease or a recent sea battle.

6) Royal Standard 

If a ship is flying the gaudy Royal Standard, be it a warship, a pleasure craft or a small boat, it means that the Monarch himself or herself his on board, and there will be high ranking nobles nearby and plenty of bodyguards for security.

7) Yellow and Red 

A yellow and red flag means that someone has fallen, jumped, or been washed overboard and the ship's crew is actively trying to find him or her. If the person is rescued before they drown there might be a reward of some kind to be earned.