I've seen this at nurseries too, and as Mitchel mentioned, I have seen 'Orinoco' growing pretty much in water with no problems. If there is enough heat and sun to pull all that water thru the plant, it shouldn't be a problem. 99% of water taken up by plants is lost thru transpiration, and they can take up a lot, especially something like bananas with such a large leaf area. The reasons plants need certain amounts of different things like sunlight, water, warmth and humidity is because they are in a struggle to keep a balance essentially. However, if one or more of these things are missing there are things you can do to help keep your plant growing. If the humidity is very low, this means it will be losing water much quicker than normal which can be helped by giving it more water. Basically, the things which lead to increased water uptake is heat, sunlight, wind and low humidity (probably a few more im not thinking of right now), so if you have some of these elements affecting the plant, than you may be able to get away with giving it way more water than it normally needs, such as growing it semi-aquatically. I think what it really comes down to though is oxygen availability to the roots. Being submerged in mud is much different that in water. I don't think you will have problems growing banana semi-aquatically if you maintain some root contact with dry soil or the air, or maybe oxygenate the water (pretty much getting into hydroponics here, which bananas can do great in). If you want to experiment, you could try just slightly submerging a potted plant, or planting at the edge of a pond as in the photos. However, most of what I just wrote is speculation (though I have seen it done), and if you try this and your plants die its not my fault!
