Yeah I agree solo boarding is loads of fun
. My only regret is that I've only now discovered that.
Oh, about the Colossus Flag I've noticed it spawns right near Outcast. As for the Man-O-War Flag, it spawns near Kingshead.
Edit: As for their course, Colossus Flag sails from Outcast all the way to Cuba I believe. And I think Man-O-War Flag sails from Kingshead to Tortuga. Keep in mind I always play on quite servers.