If it is EPDM rubber you can get a seam sealer from a professional roofing supplier. Clean seams
with gasoline and brush on sealer (or run a bead from caulk tube) and join pieces. The rubber bonds
so well that if you try to rip it it will tear next to the seam , not the seam. Allow all gasoline
to evaporate before filling with water. This sealant is used for commercial roofing and is much
cheaper than anything sold for "POND" or "RV" use. It is sold in 5 gal. buckets but they sell
smaller amounts for "PATCH KITS". Good luck, Hank