If you've got 'em in pots, pull the pot and have a look. If you see roots growing over the rim or out of the holes, it might be time to divide.
I've seen entire bottoms of ponds covered in roots, with the pots embedded in the root mass. That's neglect.
Dividing often makes for a stronger, more vigorous plant. Don't try to save every little bit, you'll just overplant. If you don't need a portion of tuber, chuck it or give it away.
Depending on the species, the dividing time may vary. Judging from your pond, and location, I'm assuming you've got all tropicals rather than hardys.
More importantly, get in there once in a while and remove the dying pads and spent flowers before they sink to the bottom and start to decay.