This Hydro Force Rapture watergun is offer at Smyths Toys at the moment. It's currently £14.99, reduced from the original price of £24.99.
This toy has a pre-pessurised cannister as well as a sliding stock and syncronised pump action - it's billed as the first quick-loading pre-pressurised water system, and although I'm not entirely sure what that means I reckon it would impress would-be water-warriors. The Hydro Force Rapture gun can fire water at a range of up to 30-35 feet and can hold up to 22 fluid ounces. It's suitable for children aged 8 years and over. But if you're thinking of buying one it might be worth arming yourself with an extra one - I suspect you'll need to defend yourself once this gets into the hands of a youngster.