During the Challenge of Poseidon in Pandora's Temple, you will be swimming in a lengthy underwater section after finding Poseidon's Trident. There are four Nyads in this area. The first one is on your normal path and is impossible to miss. Approach the Nyad and press  for a quick makeout session and the trophy. Two of the Nyads will give you collectibles (a Phoenix feather and a Gorgon eye) if you are below a certain total for each of them. If you are following a collectible guide (like you should be) then you will not receive these items.