There is a range of ways to build a water garden. Your water garden will need a lining, and it can be as cheap as a plastic lining, or you can create a concrete shell that will last forever. Expect the plastic liner to last no more than 20 years, and probably a lot less. In some cases, stainless steel has been used effectively.
Once the water garden on your patio is waterproofed, you can add water. Rainwater is the best as you might expect. If you are using municipal water, it will be chlorinated, so it’s best to wait a few days before adding plants. Well water is the least desirable because it is the least aerated, and will have to set for several days also.
As for plant, have a look around and see which ones you’d like. There are so many that it will be easy to find something that strikes your fancy. Water lilies are popular, but you should not allow them to entirely cover the surface of the garden because it will choke out the oxygen. Some people will have problems finding plants in their area, especially if they live in a desert area. There may also be a problem or choice at your local nursery. Many plants are illegal in certain areas because of the thread of bio-invasion. If they should make their way into the surrounding wetlands you could have an eco-disaster on your hands.
Water ponds add much more to your outdoor living and patio experience that they are hard to numerate. It’s not simply a matter of plants, but birds, bees, frogs, dragonflies and all sorts of other things add to the entire experience. Note that if you should ever want fish in your pond, you will have to make the decision before you start. Fish ponds need added filtration–significantly more than simple clear water ponds–to maintain cleanliness of the water as well as aerate it.