Zeus is obviously a pretty smart guy, infiltrating the home of the titans and the temple of Cronus, right under the nose of his father. Of course, Cronus thinks he’s already dealt with the boy, so he’s not exactly looking for him and he does have help in the palace from Rhea, Thea, and Gaia.
As he learns the players better, he picks out the titan Metis as one that he might be able to use. Metis is the daughter of Oceanus, god of the oceans and one of the original twelve. Eben in later times, after the Olympians had come to power, Oceanus continued to rule the oceans. Today we think of Poseidon as god of the oceans, but that’s not really correct. Poseidon is more properly the god of the sea and rives and lakes. Once you get out into the open ocean, you leave the realm of Poseidon and enter the domain of Oceanus.
As for Metis, Zeus turns on his well known charm to seduce the goddess.