I have a water lily growing just on the surface of the water too. I put it there on purpose as an experiment.
I guess it shows that waterlilies don't need to be in pots.
Sounds like yours is happier living on the surface!
You could cut the part of the tuber off that you can reach and start it somewhere else. (you are talking hardy lily, right?). I'm thinking your lily outgrew its pot and started growing elsewhere.
You better warn your neighbors. haha

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This pic was taken 2 weeks ago. This was back when the water lily looked normal. See how the lily pads are floating on top of the surface?
I hope that it did just outgrow it's pot and when I take out the big huge mass that's there now, what's in the pot will still be there!
This is my favorite water lily. I love apricot, plus it's the only one that's really been happy in my pond. I've been told that it's too deep for water lilies to be happy.

All winter I had a mass of roots frozen in the pond ice. I was going to retrive the mass of roots once the ice melted, and throw it away, but Spring arrived very quickly, and before I could retrive it, it was putting out lily pads! It's been floating on the pond surface since then, and is blooming better than it ever did in a pot.

This would be a very easy way to deal with lilies! It would be great to not have to pull out that big tub to divide or overwinter.
I wonder how this would do over several years? Sometimes plants (at least trees do this), put out tons of seeds when they start to get stressed. (Like they somehow know they might not be around much longer, and need to get out all the babies they can).

Posting some pics of this monstrosity that I pulled out of the pond! It took me and an 18 year old strong boy to pull it out- and we had to yank off roots forever before we could even do it. He had to get under it and push it out of the pond. He also found the pot I originally planted it in. It didn't look overgrown, matter of fact, there isn't much growing in there at all. He put the pot back in the pond, but I don't know if the water lily will grow back.
Here's what we pulled out:
A top view

{{gwi:206617}}Rolled over to look at root system
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At one point my brother started asking about these "octopus things" coming off the water lily. They were touching his legs in the pond. He pulled them up and they looked like this:
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Is that part of the root? There were lots of them in there.
Thank goodness it didn't touch my leg! I would have flipped out!

I think those are tubers. You could cut those up and get more plants (if you wanted to). haha Sounds like you have enough.
What was the name of the lily? How big is your pond?

Actually, I may go ahead and pot them.
I have no idea what the lily's name was. I bought 10 or so and put them in when the pond was built. Of those 10, I think there are only 3 left. And of those 3, this one was the only one with more than one or two lily pads! Now that this one has been pulled out, there is only a couple of lily pads for my fish to hide under! This monster used to take up 1/3 of the surface area on an 18' by 18' pond.
I need to find all those old pots in there where the lilies are dead and put the tubers in them. I don't want more pots sitting at the bottom providing a place for leaves and nasty stuff to gather!