The design of your pond will be determined by many factors, the most important of which are its surroundings and its purpose. There are two basic design types, formal and informal. A formal design is characterized by geometric shapes and is clearly man-made. An informal design is free-form in shape and has the appearance of being part of the natural landscape.
Consider the architectural style of the surrounding buildings, as well as the existing landscape when selecting between a formal and informal design.
The edging materials you choose and the way you assemble them will also determine the design type. A border of loose and irregular field stone, grass or plants cascading and tumbling into the water will look informal, whereas paving stones or other permanently assembled materials help reinforce the geometric shape of a formal pond.
When it comes to determining the size of your pond, both the scale of the existing landscape and the pond's desired use must be considered. Avid gardeners will no doubt require a larger surface area to accommodate a large variety of plants, whereas a less devoted gardener who only wants to have a waterlily and a few fish will not require that much space. Keep in mind that at least 60 to 75 % of the pond's surface area should be covered by plant material.
The depth of the pond is almost solely determined by its purpose, taking into account safety considerations. Different varieties of water plants require different water depths. For example, a potted waterlily prefers to have 12 inches (30 cm) between the soil surface and the water line, whereas a bulrush prefers to have only its roots in water. If you wish to cultivate a variety of water plants then planting ledges at different depths should be incorporated within the pond. The pond should also provide enough depth so that plants and fish can be protected during winter's cold.
Planting ledges have the added advantage of acting as access steps when you have to enter the pond for any reason. A pond with straight or sloped sides will be very dangerous to walk into plus the plant baskets or pots will have nowhere stable to sit. Even with several ledges in place you still have to be careful when entering or stepping out of the pond because the soil below may not be stable enough to support the weight of a person. Another thing to consider is the pond liner itself which will be very slippery.
A water garden can also be accommodated without expanses of open water, either because of safety concerns or by choice.
A bog garden is an ideal environment for moisture-loving plants and is very easy to construct. Simply dig out a 12 to 18 inch (30 to 45 cm) deep area, waterproof it with a pond liner, fill with soil and plant with reeds, rushes, ferns and irises. To support bog plants the soil should be moist at all times. Perhaps you already have a natural low lying wet spot in your garden. This might just be the solution.