Apologies if this is blindingly obvious but I have never seen it on the forums
Turn 1 Send 1 boat outside Rome and put Diplomat on it (use all boat's movement points towards Sicily)
Turn 2 Use all boat's movement points round the bottom of Sicily
Turn 3 Drop off Diplomat on Sicily (close to Greek diplomat) and blockade Syracuse
Turn 4 Offer Greeks ceasfire for 18,000 denari (they will pay) (even if Carthage has taken Syracuse) Then reblockade Syracuse
Turn 5 If Carthage has taken Syracuse offer ceasfire for 15,000 denari. If still in Greek hands wait till turn 6 and offer ceasfire for 15,00 denari
The result is a lot of extra denari without really affecting your campaign

ENJOY