It isn't really an aquatic, I've got it growing in live moss with other insectivorous plants (Pinguicula, Drosera and Dionea), and it does very well like that. I've also got a small leaved Marsilea in with them, and that also does much better than it did submersed.
It's exceptionally difficult to get started underwater. It's a magnificent plant though, it both forms as DW just proved. The best advice I received was to grow it out emeresed first then offer it ultra high co2 for the first month after filling the tank. The less you disturb it the better, if grown out in this way it seems to adapt most readily.
I don't think they get any ions (Ca+, NO3- etc.) through their "roots", as apparently they don't have any. There will be small organisms present (rotifers, diatoms, nematodes etc.), even in the very nutrient poor, acid surroundings and they will get the nutrients they require from these via their bladders, they may also use the carbon from these as an additional carbon source to augment that produced from their photosynthetic surfaces (apparently they don't have any true "leaves" either).
I read a reference yesterday where they used marked carbon (presumably carbon 13 or 14) and found that there was carbon transfer from the surrounding organic matter to the plant, suggesting that the bladders are "capturing" and assimilating this, as well as more mobile organisms (I'll try and re-find the reference).