Planting
In a pond up to 50cm deep, it's easiest to leave the plant in the pot. If the leaves are under water then the leaf stems will lengthen until the leaves are above the surface. If planted in the pond without the pot, plants can be put in soil or silt on the bottom where the pond is no more than 40cm deep.
Maintenance
Plants do not need fertilising. They get nutrients from the air and rotted plant matter in the water. e.g., Leaves that fall in.
Let the pond dry out a little over Spring and Summer to encourage flowering, and cut dead flower stems back only when you are sure there are no buds left on the stem.
Propagation
Seed collection can be a problem as some seed will be dropping while buds are still opening. Dried flower heads should be placed in a dry paper bag for a couple of weeks, then the tiny seeds should separate readily. Almost fill a punnet with sandy soil which has not been fertilised for at least 2 years. Spread seed thinly over the soil surface, then place the punnet in a container with water half the depth of the punnet. Don't let the container dry out, but don't overfill it or disturb the soil surface. Germination takes 2 to 3 months in warm weather, and when about 3cm high the seedlings can be transferred into tubes or 25 to 75mm pots using a good quality native plant potting mix. (Other potting mixes have too much phosphorus, which is bad for most Australian native plants.) Put the pots in water about half the depth of the pots. When the plants are noticeably growing larger, fill the container until the water is just above the top of the pots.