--Cut off all the green growth from this season
--Divide the root ball, reducing it to approx 1/3 of its size
--Repot the smaller root ball, adding dirt as needed
--Cover the pot with small gravel to keep the dirt in and fish out
--Sink the pots to the bottom of the pond for the winter
Oh, and also to muck out the bottom from a year's worth of dead leaves, vegetation, fish poop, etc. In my water garden this amounts to about 2 pretty full wheelbarrows of the best "soil" you could imagine.
Below is one of our favorite plants: water hyacinth. In the spring I buy a couple pieces of this floating plant and by the end of the season they have reproduced many times.
What I always find interesting is that the hyacinth growing close to the flowing water inlet (top of photo) gets MUCH bigger than that which grows in calm water (foreground of photo). Note that I have put these plants together for the photo, this is not where they grew:
[image credit Gary]