What is the pshat in the statement of the gemara that according to the one who says that the river we are talking about here is an irrigation ditch, it has to have water in it?
The hint is in Tosfos where he says that according to the one who says we are talking about a literal river, there will be an exemption even if there is no water in the river because it is deep. So despite the fact that it is not as wide as the public area, it is less likely the fire will pass over that wadi.
The irrigation ditch we are talking about is only one cubit deep, as Rashi told us earlier – this is the reason it is called an אמת המים. Thus, the depth doesn’t help us, and the prevention must be because of the water itself.