the reason the banana's seem watery is because freezing creates ice crystals, which penetrate the cells of the banana, releasing moisture....thats why freezing is ok if you plan on mushing them up anyway (baking, making banana ice cream, smoothies, etc)....the texture will be off if you plan on eating them whole after freezing
btw, the same principle applies to meats....thats why frozen steak or chicken may not be as juicy after frozen - all the juice, which is no longer contained in the cells, just runs out while cooking