Great guide

I personally have two separate diplomacys.First one is for factions near me,with them i use the old "enemy of my enemy must be my friend"diplomacy.
The other thing i do is large scale blackmail.I have lone ships with diplomats aboard.Sailing all around the seas.When i see some faction targeted by multiple enemies,i simply blockade one or more of their ports and demand money for ceacefire.
It has paid of wery well.Its not very honourable,but im playing as roman julii faction and i think it feels to me very "Roman"thing to do.