Water alone is a great UV inhibitor.
Purchase your liner in a single welded flat sheet of material. It is better to have a liner too big than too small. It is easier to trim away the excess than to field seam and make the liner larger. Field seaming presents some degree of difficulty for the average consumer and, consequently, should be avoided. Lining Materials has more detailed information on the different materials available See 1998 Price List for pond liner prices and sale items.