B. Not touch the washing water: [1]
One must be [very[2]] careful[3] not to touch the water in the vessel before washing[4] as one who does so contaminates the water with his hands [and is considered as if it was already used and is hence invalid for washing[5]]. Thus one must be very careful while washing with the vessel that his fingers do not touch the water in the vessel.[6] [See Q&A regarding if one touched the water prior to going to sleep.]
[1] Siddur; Basra 4:2, Kama 4:12
[2] Basra 4:2
[3] Siddur and Basra 4:2, However Kama [4:6] does not warn against doing so, rather just simply states in 4:12 that if one does so the water is contaminated.
[4] Touching after washing: However if one already washed, touching other water accidently does not contaminate it. That which is explained in Beis Yosef; Shulchan Gavoa 1:8; Kaf Hachaim 4:25 that even after washing one should not touch it, is referring specifically to beer, as touching it can ruin it. It has no relation to the current Halacha. [Mamar Mordechai 4:5]
[5] Kama ibid
[6] Basra ibid
What is the law if one accidently touched the water: From Admur ibid it is implied that even if one accidently touched the washing water it becomes impure. However the Ashel Avraham Butchach 4 rules that only when one touches the water intentionally does it become impure as opposed to one who touches it by mistake. [See Piskeiy Teshuvos 4 footnote 158]