OK, here's an endorsement for an incredible product, and they don't even give me free glue!
They sometimes call it Wet-Bond and sometimes Underwater Glue but it all comes from a company called Mr. Sticky -- www.mrstickys.com. Actually the whole range of Mr. Sticky adhesives are two part epoxies. But they do have some unique claims to fame.
They can be applied underwater.
They are non toxic to people or fish.
They will glue to most plastics, including PVC -- which includes most garden furniture and swimming pool parts -- even butt welding PVC pipe.
They will stick to metal and wood.
Some are rigid, some are flexible depending on your needs.
Most adhesives say to apply to a clean dry surface. It is almost the same with Wet-Bond, except for the dry part. The real key is to take a nylon cleaning pad or even steel wool to really scrape the surface clean -- even if you are working under water in a pool or a rain gutter. That will both give the epoxy access to the surface itself, but it will rough up the surface just a bit to help create a mechanical bond. When you apply the epoxy, use a stick to work it onto the surface. Actually what you are doing is pushing the water aside and where the epoxy and the surface come into direct contact, the water will not come back between them, so work from the middle out, or from one side to the other literally squeezing the water out from the joint. If you are applying it where the water pressure wants to push it off, give it mechanical support until it cures -- not so necessary on small holes if the water is trying to push the epoxy into the hole.
OK, it is not cheap and it is often hard to find -- try Home Hardware, swimming pool stores or the company itself on the web.
Lepage has seen the demand and now has brought out a Marine Epoxy that can also work underwater or on wet surfaces -- but not flexible surfaces and not all plastics. However you will find it conveniently in most renovation stores that carry LePage products.