DD is 3 and won't drink plain water unless very thirsty. I give her pure juice, slightly watered down but am aware it is potentially teeth rotting. The low sugar squashes have lots of nasty chemicals in it seems including sacchirine or aspartamane - some.
Got some still flavoured water which went down well today but it still has some scary additives in - well they read scary anyway!
I'm sure she would drink water if there are no other options ! Try adding a splash of fruit juice and diluting it, then increasing the level of dilution. Juice at meals is ok but try plain water in between.
Would like to add that I don't like drinking water unless I am very thirsty/it is ice cold water and I will go without a drink if it's the only thing available I know it's bad but I really don't like it that much.
OP have you tried High Juice squash as well? I think that is free of aspartame etc.