Hottonia

[kyle at onebox_com]

That link does have pictures of H. palustris.  At least in the wild 
(aside from growing on different continents) telling them apart
is easy.  H. inflata, as the name suggest makes bizarre inflated
stems that remind me of long narrow party balloons.    You should be able to tell right away if the picture really is inflata
check out this link:
http://www.usi.edu/science/biology/TwinSwamps/Hottonia_inflata.htm

I'm not sure if there is a reliable way to tell them apart in 
nonreproductive state if inflated stems are lacking.  I'd love to 
know if anyone can do it though!