There are two main ways that something may become radioactive, being contaminated with little pieces of something radioactive (for instance nuclear fallout), or

There are two main ways that something may become radioactive, being contaminated with little pieces of something radioactive (for instance nuclear fallout), or through neutron activation, in which neutron radiation creates radioactive isotopes in whatever it hits. Assuming the fuel rods have intact cladding, then the first shouldn't be happening in a cooling pond. However the fuel rods do give off neutron radiation, and a small amount of the hydrogen in the water is turned into tritium by that neutron radiation. Tritium is a weak beta emitter, it could harm you if you drank a lot of it. But it's not a long-term problem because it has a half life of 12 years.