The Athenians can win at sea easily enough. A victory at Amorgos (or, really, the Hellespont, since the first battle is what allowed the Macedonians to cross) against Cleitus The White is certainly a possibility-the Athenians had somewhere in the vicinity of 170 ships at their disposal (they actually had around 240-400 ships available, but only had the manpower for 170) compared to 240 for Cleitus, but the battle was by no means unwinnable.
The problem however was the Athenian commander, Euetion. The circumstances surrounding the battle (i.e. Athens barely suffering any losses yet still fleeing in defeat) suggest Euetion sided with anti-war elements in the city. It is possible he struck a deal with Cleitus. I'm not sure who you replace him with-Athens had no shortage of fiery statesmen in this period, but they were distinctly lacking in generalship. Perhaps another mercenary commander can fit the bill, and pull off a stunning victory.
Have that happen, and the Macedonians can't cross. At that point, it's a waiting game. Once things go to hell in a handbasket and the Diadochi start eating each other alive, Athens is suddenly a valuable ally to an aspiring dynast if they play their cards right...