Margaritaville is about as American as the "Outback Steakhouse" (a restaurant chain in the states that sells fosters and kangaroo) is an accurate representation of Australian culture. It's kitchy and feels like a beach bar off the Florida keys, with ten different kinds of margaritas, a decent selection of TexMex, coconut deep fried prawns and other American style food greatness.
Margaritaville is in a prime spot in darling harbour looking out at the South Steyne ferry and across at the city in all it's glory so it's a great spot to sit with a fried or a date and watch the world go by. Especially from the seaward-facing elevated two seater couches along the waterfront side!
The pre-mixed margaritas are American in their strength too so there's no need to spring for a more expensive or freshly made one and while not super cheap, they're inline with mid range sydney cocktail prices.
The fajitas were enough to satisfy two of us and make a good compliment to a drink and there's plenty to do once you're in darling harbour so check it out!