Here is a picture of a container water garden and suggestions for plants. You can create one using a filter and water pump or simply create one with just water and plants like this one. This container is approximately 18 inches in diameter and about 10 inches deep. The dwarf water lily and umbrella palm are planted in smaller containers and then the water lettuce and azolla just float on top. Make sure you seal containers on the inside with a concrete sealer or spray urethane and seal any holes in the bottom of the container with clear silicone caulk.
Lotus are very popular water garden plants that are grown in containers. Other plants to consider would be any dwarf or small water lily, dwarf cattails, dwarf papyrus, sweet flag, pickerel rush, umbrella palm or any smaller size plant. Consider using floating plants such as azolla, water lettuce or water hyacinths which will cover the water and help keep it algae free. Using azolla or duckweed creates a blanket of coverage on top of the water.
If you choose to use a larger container and want to add a small pump we recommended a 65 gph pump. http://www.dragonflyaquatics.com/pondmaster-65-gph-pump.html. You can even add some small fish then.
There are endless possibilities when creating a water garden feature. So whether you choose to dig a hole in the yard or one that wishes just to have a small water feature to enjoy, its heaven to simply bask in the quiet tranquility of your own creation whatever it may be.