G'day Adam,

We dont carry any spare parts for the Watercells, but yu may be able to grab one from Sea to Summit directly.

G'day Paul,

Whilst we haven't tried out a wide-mouth Nalgene and Watercell combination in person, we can confirm that both have a 63mm thread size which is generally universal - also the same as the MSR Dromedary water systems, for example.  Based on this info we're pretty confident you won't have issues.

G'day Nicole,

Ofcourse.  Just like with and soft water bladder, you can carry as much or as little water as you like.

G'day Jovan,

We've had this question once before on a different size Watercell. 
I'll post our response below:

"There's a few of us in the office with these Watercell X bladders, and only one of us has had issues with taste.
Ben (who has had the taste issues) uses normal tap water, whereas the rest of us use filtered water.
We have pondered whether or not the added chemicals in the tap water somehow interact with the Watercell lining. No other explanation really!
Ben has had some success cleaning the bladders out with bicarb and lemon juice, which appears to have remedied the situation.
Hope that helps!"

In addition to that, we've had these Watercells for a fair while now and it seems that the taste/smell only affects a handful of people.  It may come down to water type or sensitivities, but all feedback reports that this does go away with time.