Wakahisa can drain chakra from others if (a) both parties are in the same zone and (b) both parties are in physical contact with the same water supply. Water-walking counts as physical contact for this purpose, due to the constant emission of chakra required.
Additionally, while draining someone the Wakahisa will gain a rough sense of their current reserves in relation to other people that the user has drained in the past. E.g. "A little bigger than Hazō's."
Mechanically:
As a Standard action, roll (your VD skill) vs (their Resolve).
You gain N chakra, they lose N chakra. N = (your VD level + difference between the two rolls). N is capped: 0 <= N <= 2*VD
You cannot drain more chakra then they have.
The target may deliberately fail the roll if willing, in which case treat their Resolve as 0.
The target always knows that they are being drained and which direction the drain is coming from.
The Wakahisa must have a supply of liquid water to put the chakra into in order to drain. The only exception is if they retain control of the chakra for immediate use on the same turn or the next. Any action other than spending the chakra will cause it to dissolve into the environment, as will taking stress from any other source while holding onto it.
The water must be actual water, not a chakra construct.
You cannot retain more chakra than will fit into your Capacity. Any chakra drained above that point simply dissipates.
Note that as per the Requirements for Water-based Chakra Storage section if you drain chakra and store it in uncontained water then it will disperse at the end of the following turn. It is, of course, possible to move some of your chakra into uncontained water specifically to let it disperse, although it's rarely something you'd want to do.
Example:
Noburi (VD of 34) drains Alice (Resolve 28). They both roll average, meaning that Noburi gains 34 + (34 - 28) = 40 CP.
Noburi (VD of 34) drains Zabuza (Resolve 57). They both roll average, meaning that Noburi gains 34 + (34 - 57) = 11 CP.
You may divide your maximum drain among as many targets as you like, making a separate 'VD vs Resolve' roll against each one. You must declare how much drain you are allocating to each target before rolling; this amount is a cap on how much you will drain from them if you succeed. (Hopefully we don't actually need to say it, but: you must allocate your drain in terms of positive integers. You can't, for example, drain -0.5 + 3i CP from someone.)
A secret technique of the Wakahisa is the ability for highly skilled users to drain through mist instead of liquid water. This works exactly like draining through liquid water except that you lose 5 points from your roll.