Are their any universal signs of not wanting to fight?

Be aggressive.

Insanely aggressive.

You see a ship, no matter what the circumstances are or how far away it is, pin you’re ears back and attack!

Head straight for it!

Believe it or not it works. Sure they may be able to kick your arss but no one wants to deal with crazy.

Even Galleons will avoid Sloops.

You never know what the other guys are doing, how much time they have put in and what they have on board.

Charging at everyone like a maniac works.

If a Gally has an Athena and has been questing all day you think they want to fend off two sweaties with nothing to lose charging at them in a fresh spawn default skin sloop? Hell no they don’t.

Now sit there like a victim with your anchor and sails down looking like your on an island and even the meekest of players will take that opportunity to attack!

If you want to show that you don't want to fight, maybe raise the white flag and put your cannons aiming at the sky. Also, playing music to another player, raising your tankard or raising your lantern means you are friendly, though the music is the most used and well - known technique. If you do want to fight, I agree with @Glannigan. Don't be afraid of the four -man - galleon in the distance, and don't be afraid to lose your loot. It's just loot, you can get the same amount of it again if you try, and it is just use to improve a meaningless number in your stats, and you can only use the gold to buy cosmetics. Don't be salty or sad when you lose and don't give up PvP cos you didn't win your first few battles. The most fun thing to do in the Seas is fight, have legendary adventures and have fun with your friends, so don't give up on it. Practice, and you will learn to fight perfectly.

You'll get attacked and you'll lose loot and it'll be rough for a while

but just keep it in your mind that most of us that have millions and millions of gold and most of the commendations done started out in your shoes.

This game has more good and bad luck in it than anything else. You will have frequent run ins with both

The best way in my view isn't to waste time trying to figure out how to prevent a pirate from pirating the best way is to be efficient when the gettin' is good. Waste as little time as possible. The more efficient you play the more you will gain and the less you will lose even with hostile pirates out there.

Sell often. The less you carry the less you have to lose

Pacifists need to play smart and efficient but they can still thrive in dangerous waters.

My suggestion is stick to megs and skelly ships and short island visits for a while. You want to keep moving so you don't get tucked on or snuck up on. This allows you to gather plenty of loot without the vulnerability of island wandering

As you get more experienced you will make a lot of gold so don't worry about some losses now. It'll be frustrating but you more often than not won't be losing much at your current stage compared to what you'll bring in down the road.

universal sign of "i don't want to fight" is dying quickly.
some (a lot) of people are insanely aggressive in this game and will attack on sight whether you're a threat or not. there could be some sort of emote that made you unable to attack for 3 hours and beamed unmissable light out of the top of your head and you'd still get attacked and killed just for being too close.

playing music is often a good sign, but basically you can't trust anyone, and no one will really trust you. you're safer in a group, but safer doesn't mean safe.

also keep an eye on your map. if someone is purposefully looking for trouble they might be flying the reaper's mark or sailing as the reaper's emissary. if they're doing either of these then they will show up as a shadowy ship on your ships map and you can avoid them.

like others have said: loot's not yours until you've sold it so make sure you pop in to sell things whenever you're passing by an outpost. just be laid back and relaxed about the game. if someone sinks you, you spawn far away from them. if you find you're constantly getting swamped then log out and in and you'll be on another server with different players (who may not be so pvp inclined).

if you find yourself boarded and spawn camped, then remember you can scuttle your ship while sitting on the ferry of the damned (this is the one that annoys me the most - when they don't just kill, sink, and move on but rather play with you by setting up on your ship covering the spawn spots to shoot you in the back while you're loading in. maybe i'm bad but that's super difficult to get out of especially when you're outnumbered)

and finally - if you run into the toxic guys that follow you around swearing at you then record a clip of them doing it with the little mic icon above their head (win + alt + R on PC to start a recording or hit the xbox button and choose capture last X seconds on xbox) and report them. we don't want those toxic peeps around.

@stonedhippieman
Pro tip: Do not and I really mean DO NOT wear the Ocean Crawler or Kraken Gear! Just trust me on this every streamer and every pirate that has been playing a long time will see that and be like oo newbie I am gonna stomp on them. So ya just save yourself don't wear it