Just an alternative suggestion.
Tim is right , if you need TOTAL waterproofing (for photographic equipment or such like) then the Peli cases are the business, however they are expensive for their capacity.
I use a semi-submersable dry bag, which is much cheaper, a 60 litre capacity bag works out at around £55. , and keeps out all that the UK weather has thrown at it so far. As long as it is correctly sealed (and their system makes this quite easy) it is claimed that they will withstand a short total immersion in water. I have not tested this claim (yet!).
They are made by Overboard
http://www.over-board.co.uk/duffels.html I use their small pack for keys and a phone, carried in my buoyancy aid, and I can guarantee this is totally waterproof (having spent several minutes swimming in a particularly retentive feature when I should have been kayaking) I also use a larger one in the back of the boat for food, drink and spare clothing which again is totally waterproof.
