In reply to mypyrex:
Rodents etc. can chew through a dry bag and clipbox tupperware is heavy. If you use a dry bag wash the outside thoroughly after packing and keep away from any food until stashed so there are no food odours on the outside.
Some points if you are going to do this.
1) In a "wild" environment it should be absolutely discreet. Nobody should be able to happen upon your gear. If they can then you haven't stashed, you have littered.
2) You should absolutely 100% be able to reclaim it. Even if you have to change plans due to weather or injury you go back. If you don't it is littering.
3) On the odd occasions I have done this in other countries (I am not convinced anywhere in the UK is really remote enough to warrant the effort when you account for the weight penalty of packing out the weather/animal proofing) I have used a document wallet on the outside of the bag detailing why it is there and when I intend to collect it indicating that any sizeable date past this means something went wrong and it is swag. When I stashed water this way I left a little extra for anyone that happened upon it by way of an apology/bribe not to take the lot