Plastic is used for water lines because plastic is clean and won't pollute the water or leach into the supply. However, if your plastic water line springs a leak, it can be a huge issue. This is especially true is you don't want to replace the line entirely, but you need to fix the leak that's sprung. Fortunately, all you need to do is get the proper sealant, often used with engine lines, to plug your leak.
Turn off the water pressure to the line and drain it out. Open up the line and let all of the water out into a bucket or some other container. Examine the area that's leaking to be sure it's small enough to be patched.
Let the area to be patched dry completely. Put your safety gloves on and apply your sealant to the leak. Make certain that it's setting well. Wait until the sealant is completely dry.
Close the plastic so there will be pressure again and open up the valve. Water should flow through, right past your seal of glue. Make sure the seal holds.
Items you will need
If the water you're sealing is going to be for drinking, make sure the sealant you choose is applicable for drinking water. Otherwise, it will be a health risk.