Important note: This method can ONLY be used prior to development taking place. If you attempt to raise or lower land levels during an existing game you will destroy ALL development.
The following method will enable you to produce sharp edged waterways to your city map prior to development. In the first example we will create a straight edged coastline. You will need smoncrie's hole digging lots in order to achieve the desired results.
Go into God Mode either before establishing your city or after by clicking ctr+shift+alt at the same time as selecting God Mode and raise the level of the whole map by a single click only.
Switch back to Mayor Mode and select the 26m monorail hole digging lot and lay down as many as you need in order to provide an area large enough to provide you with a workable area. It is not necessary to lay down the hole digging lots for the entire expanse you wish to create as you can easily expand your original selection by using single street or road tiles..or by using the Mayor Mode levelling tool for larger areas. I tend to use a combination of the two...levelling tool first and then tidy up the edges with road tiles.
Once demolished and tidied up with road tiles, you will be left with an area of lowered terrain as below:
Now, go back to God Mode again and lower the terrain one click. You will now have your first stretch of straight coastline and you will be able to check the depth of the water. As you can see, laying down a single 26m monorail (which is the deepest in the set) produces only a shallow water depth. If you want deeper water than this you must repeat the process and each hole digging lot will result in a different water depth.
Here, I have used the 26 monorail lots twice in order to create a deep water coastline.
Obviously this kind of coastline is unrealistic not only because of its absolute straight edge, but also because of the sharpness of the land edge. However it has many uses, some of which will be covered in due course.
Now you have the basic concept of creating the coastline, we shall look at ways of using the method in your cities. In this first example I will create a level beach using the standard in-game beach lots. In order to reproduce this design you will need a slope mod. In this instance I am using the slope mod created by bones1, but other variations will work just as well.
So first of all, create a straight coastline as below:
Now, go into God Mode and raise the level of the map by one click. Once you have done this, draw a road on the first raised tile of the shoreline along its whole length.
Next go back into Mayor Mode again and draw a road straight down from the raised to the lower level at each tile. Because you created a road on the upper tile, the slope will extrude giving us a "raised" descent to the lower level. If you do not draw the upper road first the slope will indent and push the upper level of your shoreline back.
Repeat this for the whole length of your shoreline.
Now go back into God Mode and lower the level of the whole map...incidentally it is not necessary to demolish the roads you have created as they will be destroyed when you lower the land level. You will now have a sloping shoreline.
Add the in-game beach lots to your shoreline and you're done.
You may feel that the slope of your beach is too high and if so, we can easily remedy this by lowering the level of the land for a few tiles back from the water's edge. Again, using smoncrie's hole digging lots (this time I have used the sunken version) lower an area of tiles from the first raised tile back inland. Remember, each hole digging lot will produce different results. Experiment to find the height you require.
As you can see in this example, once you have lowered the terrain the land sits much closer to the water level. Once you are happy with the height of your shoreline, you can raise the land and create your slope using the road method described previously.
The result is a shallower angle on the slope of the beach as you can see in the comparison image.
So, thats the basics covered for your coastlines. Now, you can also use the same method for canals, rivers, harbours and ports. Lots included are by jeronij and marrast.
And finally, if you have the imagination, lots of spare time and nothing else better to do, you can also go regional !!!