Alton Towers have started to allow the purchase of the annual pass to the Cariba Creek Water Park.
This was advertised on the Alton Towers website earlier in January - and it was even available to purchase - but was then removed.
At the time of writing this, the site now states 'launching in February', here:http://www.altontowers.com/pages/offers/alton-towers-annual-pass
However, we were able to purchase the annual pass on Sat 14/02/2009.
We had to go to the theme park to buy it - as they take a photo at the time of issuing it.
The waterpark staff indicated that they were to have the equipment installed on Mon 16/02/2009 (I can't confirm this).
We had tickets for the theme park - so I don't know how you're supposed to gain access to the annual pass membership office if you just want the waterpark pass.
It's worth knowing about - particularly if you've booked a hotel room following another HUKD post (we did)!
Sorry I haven't further info., but as a guide we paid £96 for a family pass for 2 adults and 1 child, and the site states "from £39 for an individual".
EDIT: changed link to point to annual pass page.