When made an alliance you get 50% from the gold and doubloons your alliance member earns. BUT THAT'S NOT IT!
The best feature, nobody ever mentioned, maybe nobody knows, is that skellie ships sunk by an alliance member count towards the legendary commendation for 500 sunken skellie ships...no matter if you shot that ship, no matter if you've ever seen it!
I wish more ships would allie up with me, so I can get the legendary cannons faster...sadly those commendations came that late into the game, I could've finished them already if the sinking from "Cursed Sails" would have count