Marginal plants are those which grow around the margins of the pond where the water is shallow. They usually have their soil and crown underwater, and sometimes their lower foliage as well. They are generally placed on planting shelves within the pond. In order to be considered a true marginal pond plant, the variety must be able to tolerate waterlogged soil or water over its crown all year. A plant which will tolerate permanently moist soil but will not tolerate water over its crown or foliage, is considered a marsh plant.
Planting depths mentioned on these pages refer to the depth of water over the crown (growing point) of the plant. This is about the same as the depth of water over the soil level. So a plant with a recommended planting depth of 0 - 4 inches, should be grown anywhere from waterlogged soil (0 inches) up to 4 inches of water over its crown.
You can grow most marginal plants in drier conditions than this if you want - many will grow in a standard flower bed, provided that the soil is not allowed to absolutely dry out. However, you should not put a plant in deeper water than the recommended maximum.