Water Transport Tiles are very costly to build. Let us imagine that you want to join these two water pools with a canal:

Often you won't have enough money to make this a real canal, so what can be done?

Creating a pseudo canal

Firstly, lower all the land between the canals but one point on each shore. This won't cost much, as you do this kind of thing very often, like when you build a train junction.

Now, lower the two remaining dots and see the water flowing into your canal. A key point here is to make sure you have the canal exactly as you want it (minus water) before you lower the points. It is much cheaper to terraform dry land than on water, and if you end up changing your mind and having to infill water, you may as well have gone with real canals in the first place for what it will cost!

Voila, you've created the canal in a much cheaper way!

You could also have made it wider, if you wanted to.