Why build an aqueduct...

[ShinigamiKenji]

If my city is next to a river do I need an aqueduct?

Only if you play as Rome. Their Baths give +4 Housing and +1 Amenity, at a time when both are scarce.

If your city is next to a river, a Granary and a couple improvements are enough to make it reach 7 pop already, which means 3 districts.

 

[DrCron]

I never build aqueducts (unless if playing Rome) and hardly ever build more than 2-3 neighborhoods in one game. If you settle correctly, housing shouldn't be that big an issue. You don't need more than 10 pop in every city to get all the districts you need to win the game. And since specialists are useless in this game, growing more than what's strictly necessary has pretty much no extra value.

 

[CoccoBill]

Personally I think the logic is pretty straight-forward. For cities without any water access it's mandatory, for coastal water it depends and for fresh water cities there's generally no point.