This sewer is not only decorative, but also useful, because you can transport items in it.
How to design the shape of the sewer is, in principle, a matter of taste. The important thing is that it contains a channel in which water can flow. After 8 blocks of flowing water, you let a block of ice in the ground, so that items are guided slithering towards the next block. The next block is here at best again water, the items 8 more blocks carries etc...
In order to prevent the water from flowing back, signs, ladders or, as in my example, half-stepping stones are installed.
You can connect inlets anywhere, which are represented on the overworld by fountains into which you throw items, or outlets from automatic farms, etc.
The sewer then ends, at best, in a storage room where goods are funneled into chests.
I think this system is a nice alternative to rail systems, even if it looks a bit dry without a texture pack.