My understanding had always been that if your city is adjacent to fresh water and a coast, you only get the +3 bonus for the fresh water, not the +1 for coast.
I did a little experimenting (starting in the Renaissance era for quick access to settlers and the aqueduct).
As Australia, I settled on a coast with a river on the inland side of the tile. (That is, the river paralleled the coast 1 tile away).
That gave me 8, which I believe breaks down as: 2 (base) + 3 (river) + 3 (coast).
So it appears you don't get the standard +1 for coast if you have fresh water, but you do get the +3 bonus for coast, regardless of whether you also have fresh water.
An aqueduct brought the total to 10, so apparently added +2 as it does for other civ's cities with fresh water.
An Australian coastal city without fresh water access gets 6, which is presumably 2 (base) + 4 (1 + 3 coast).
An aqueduct brought the total to 9, so apparently added +3 as it does for other civ's cities with only coastal water.
So it looks like the aqueduct gives an Australian coastal city with fresh or only coastal water the same bonuses it would a normal civ's city with fresh or coastal water, respectively.
As Scythia, settling on the coast at the mouth of a river got me 5: 2 (base) + 3 (river), but not the additional +1 for coast.
I didn't find a spot with a river that parallels the coast 1 tile away as I did above. I don't know if that would make a difference.