Recently, I completed a couple of units of Star Wars Legion Shoretroopers and chose to base them along the water's edge. This isn't a difficult process, but it does take a good deal of time mostly because of how long drying takes between steps. Below is my method for making realistic water bases.
This is my final result using the steps outlined below:
I undercoated the base by painting two tones of blue and roughly wet blending them together. A single color could be used as well if you don't want to deal with blending.
The painted base:
Using Vallejo Water Texture, paint on the water. I used the transparent version of the water texture, but they do make color tinted versions too. It's an acrylic based product, so clean up is with soap and water. I use an inexpensive craft brush to apply it to the base.
This will dry clear and glossy in about 12 to 24 hours. While it's still wet, you can shape the water. Since these bases are to look like the water's edge at the beach, I wanted to have some ripples and waves rolling onto the beach. To create the look, simply use a toothpick to shape the waves. Put the toothpick on the base and slowly pull it straight up out of the water texture. Repeat this to create parallel waves washing onto the shore.
After using the toothpick to create the ripples and waves, the wet base looks like this:
When the base dries, it will be completely clear but will have 3D ridges that can be drybrushed with some off white to look like cresting waves and ripples. You can also add some splashes around the figure's foot by using the point of the toothpick to pull up some of the water texture around the foot.
The final results look very believable - for these I was going for a Caribbean Sea feel with the lighter blue colors, but the same techniques will work with darker blues, or even muddy green rivers.
I hope this brief tutorial was helpful; give this a try and create some great looking bases for your figures.