Division: is with a part of the root system & stem/leaf.
divide every 2-3 year & takes 8 months of a division to mature....

I have great propagation returns with even small eye buds of various waterlilies. My viviparious lilies obviously gives me little babies of the leaf margins, but my rhizomes from my oviparious lilies generate anywhere from 6 to 12 eye buds and division a year, and I don't have to wait 2-3 years to propagate....Water temp, onset vigorous growth, and a little work go along way for many babies.

Thanks for your responses. So, if I divide the water lily, I have to include roots with the new potting, leaving the very long stem dangling out of the pot?

try the linked forum ...

Here is a link that might be useful: pond

Water lilies are so tough that even if you cut the stem off if you are dividing a well established root it should grow right back (especially if you stick a fertilizer tab or 2 designed for water lilies in with the root). I usually wait to divide my lilies until they are dormant which is what is recomended but in Florida you might be able to do it anytime. In my opinion, though beautiful, water lilies are pretty weedy.

Though I only have temperate zone lilies so maybe tropicals are more picky.

Good Luck