Oxygenating plants are those whose foliage is fully submerged in the water. This means that during daylight hours they absorb carbon dioxide from the water and release oxygen into it, hence the name. However, their main function is to combat algae and blanketweed, since they absorb nutrients from the water, and provide shade, leaving less food and light for algae to grow.
Oxygenating plants will grow at any depth where there is enough light. Most have their roots in soil, but some float freely, absorbing all they need from the water.
In recent years, due to concerns about the spread of invasive species, there has been a move away from exotic oxygenating plants towards native varieties.
Demand for oxygenating plants is highest in spring when people are starting to plant their ponds, but the best availability is actually from June onwards, since they don't really grow strongly until the water warms up.