Worldbuilding is one of the funnest parts of writing fiction. If you’re writing in the real world, the geography is easy, but if you’re inventing a new world, you have to consider a lot more things. One thing that is impossible to overlook for carbon-based (organic) lifeforms is water. There is no human civilisation that doesn’t have access to water in one form or another.
Life
Life refers not only to drinking water, but also water to irrigate crops and hydrate animals. We need it to drink, but we also need food, and our food needs water.
Transportation of goods
For much of history, the fastest way to move large quantities or heavy goods was with waterways. Shipping over the ocean or even sending boats downstream are much faster than dragging or driving over land.
Travel
Along the same lines as shipping, water can move people as easily as cargo.
Geography
The settlement of villages and towns is always determined by water sources. Nit just for the above reasons, but also for protection or even aesthetics. Large bodies of water can provide buffers against enemies or good vantage points, or even a lovely view that people want to look at for generations.
Industry
Water can power mills by use of waterwheels, it can provide water used for production, and it can carry waste away (though, we’re discouraging that, these days).
Fishing
The water itself can provide a source of food and commerce in the form of fishing. Freshwater lakes and rivers and the ocean all have distinct ecosystems that humans can take nourishment from.
Bodies of water
You cities need water. They can’t survive without water, and there are consequences for limitation of water. The island of Santorini in Greece obviously has access to the ocean, but they have very little rainfall and freshwater. They had to adapt the way they grow wine grapes to absorb as much of every little drop that they can, and they have bottled water shipped in for drinking, as a result, bottled water is crazy expensive.
But they have the sea.
You cities may be coastal, or have access to lakes or rivers, be in a place where they have heavy rainfall, or huge underground reservoirs with lots of wells for access. Whatever you do, start with water, and build the culture around how they utilize it.
Avon Van Hassel 