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As long as their leaves are out of the water they are fine. I have one thats grown to be about 3 feet tall and its even shooting off a new seperate plant at its base (the lucky bamboo)
One of my other big lucky bamboo died because after over a year my fish decided to eat a hole in it. I salvaged two cuttings from the top that had their own roots, one didn't make the acclimation one did and is growing fine.
They can grow fully underwater but only for so long before they eventually die. unless they breach the surface.
I did some research on lucky bamboo before I added it to my aquariums and there is a lot of misinformation online about it being poisonous, which is just simply not true, I've had them fully submerged except for the leaves for 2 years and the growth has been excellent.
Heres a few pics of my big guy on my 20g
I'm sure you can tell by the shape, it used to be in a 120g 2ft tall aquarium, thats why the leaves are so high up lol
good luck with your bamboo!