Compare to my Memphis in single player in 1760 AD. 4 workshops, 6 watermills, and 1 ivory with camp provide hefty production output. In addition levee adds +1 production for every river tile. Hmm... City is getting to the point where unhappiness is going to be obstacle. Maybe I should replace unused farm with yet another workshop in order to limit the growth...
[edit]
I just realized something. Flood plains gives hefty food output, but its unhealthiness can negate part of it. So:
3 food - 1 unhealthiness = 2 food
So at the beginning there is no significant advantage (or disadvantage for that matter). But when health improving tech starts coming available, cities around flood plains can support sizeable number of cottages/towns, watermills, workshops, and specialists. Has worked for me in single player twice.