If you have a pond without fish that have a penchant for plant eating this is optional. Gravel may help with initial clouding of the water as you lower the newly potted plant into the water. The gravel will prevent half-hearted attempts by the fish to play in the mud. For koi it is recommended that you utilize gravel that is bigger in diameter than your largest koi's mouth. This will prevent playful koi from throwing your gravel on the floor of the pond.
Water plants grow quickly.
As with terrestrial plants, after a season or two aquatic perennials will benefit from division. As they grow their roots become crowded in their pots. You will want to divide them. Division is important as it affords you the opportunity to examine the plant and its root systems. Plants almost always do better when their soil is refreshed and they have room to grow freely.