There are couple of plants that are very effective when used on their own in a small water garden, but the most usual approach is to scale down the planting that you would use for a conventional pond. Use a mixture of marginals, deep water plants, submerged oxygenators and floating aquatics.
The number of the plants that you will be able to accommodate on your container water garden depends on the volume and surface area of the water. In a container that can hold 50 litres of water you will have room for one dwarf or pygmy water lily, three or four shallow marginals, one or two plants of a non-invasive oxygenator and a floating aquatic.
Make sure you positioned the container correctly before you fill it, as once it holds water and plants will be too heavy and difficult to manoeuvre. The place that you choose for your container must be firm, level and close to a water supply so that it can be topped up as necessary. The ideal place is sheltered but away from trees and in sun but shaded during the hottest part of the day.
When planting your water garden container, if this is a wooden one, first pre-soak the barrel to swell the wood and make it watertight. Set the container on a level base. Cover the bottom with a 5 cm layer of washed gravel. Fill two thirds full with water. Plant aquatics individually in ordinary garden soil in plastic-mesh baskets lined with hessian to prevent the soil washing out.
Place the plants deeply in the baskets, work the soil round the roots and firm gently so that the finished surface is 2,5 cm below the rim. Top dress with washed gravel. Stand the containers of marginals on bricks so that the plants can reach the water surface.
Finish filling the container slowly to avoid disturbing the compost in the planting baskets to within 5 cm of the rim. If you will use floating aquatics in the planting design, add them when the container has been filled.