Like others have said the best thing to use is a liner. But you have to be careful what one you buy.
Butyl rubber is about the best and these days the better dealers will give you a lifetime guarantee. Get sometime like 1.00mm thick and it should cost you around 40p per square foot. Aqualast is a good manufacturer.
A cheaper product is Polypropylene (around 30p per square foot) which also comes with a lifetime guarantee.
Which ever one you choose make sure you use underlay to protect the liner.
Also try not to leave any of the liner showing as the sun does not do it much good.