So my friends and I are doing one of those Captain's Quests with the golden key compass.
We pass a player Galleon (We are a Brig) looting a shipwreck and fishing near the island we needed to land at, and I just shout through the trumpet item 'We are friendly!' as we go by.
So we do our thing, have a hold with some treasure in it, and as we are preparing to leave; one of the Galleon players swims over to our ship (And this had to be a 3-5 minute swim).
I'm bracing for an attack, but he emotes.
So we're just playing music; another player swims over.
NOW I think an attack will come; nope! ...Well...no-ish!
They raise our anchor and take the helm without a word. Me and my 2 friends are laughing and watching the entrance to the hold.
They sail us near their ship, which firebombs us. 'AH!' We think. 'TRAITORS!'
But...the 2 on our ship rapidly put out the fire, and their friend apologizes.
We then spent 2 hours as an Alliance, just fishing and playing music.
Heck, when I went to row a Mermaid Gem over, my friend dies to a shark while holding a 2nd gem; this crew grabs it and swims it over to my rowboat for me to drop off at the nearby Trading Post.
At the end, as we decided to log off, we realized we had treasure still, and gave it to them. Just started swimming treasure over to their ship.
Not every day in Sea of Thieves is cutthroat.
PS: I should mention that these guys had the angler figurehead with the fish swimming in it, so they clearly love their fishing! I understand that's a grind and a half to get?