Oxygenators are those plants that are totally submerged normally and would die if allowed to dry out. Some root into soil and other just float around in the water - without roots. To allow water garden plants of this type  to remain better submerged attach the bottom of the plant to a pebble using an elastic band or something similar.

Oxygenators as a class of  water garden plants assist in keeping plant water clean and clear by absorbing nutrients. Potted oxygenators include Water Crowfoot, and Water Violet. Monkey tail, Hortwort, Parrots feather are also typical water garden plants that act as oxygenators.

Other categories of water garden plants differing from the oxygenator class can be broken up as follows: 

Marginal water garden plants such as Iris, and Arum Lily which like to stand in water up to about 15cm (6”). Some can grow quite tall and be blown over by the wind so you may have to weight the pot or crate down with stones.

Shallow water garden plants or water garden bog plants like Marsh Marigold, Iris, Water mint. These plants like water about 5 cm (2”) deep and are good examples of water pond plants.

Deep water garden plants the best known of which is the water lily – most people spell it water lilly. Please forgive me if I skip between the lilly ponds and the lily ponds, the water lily and the water lilly.

Floating water garden plants with their hair-like roots that protrude into the water and need no soil at all. These plants certainly assist in keeping water clear since they absorb nutrients from the water and do a good job of oxygenating lilly ponds. Water Hyacynth, or the water soldier plant are examples of this type of water garden plants.