Yesterday was about how Poseidon exemplified the values of his pantheon and cultural origins. Today is about how he relates to other deities and pantheons.
Around the Mediterranean basin, it was understood that the deities of the sea worked in partnership. This is illustrated in several ways. One of which is during the Roman period where the Romans identified their own Neptunus with Poseidon and worshiped their deity at Poseidon’s altars.
Another is, interestingly, found in both Jewish and Christian scriptures in the stories of Jonah and Paul. Both of these “holy men” initially tried to run away from a request of the God of Abraham. In each of their cases, there was a great storm at sea and the people on board the ship were asked to pray to their own gods to calm the storm. Although, ultimately in the stories, it was the God of Abraham who calmed the storm it is interesting to note that in both cases it was decided on board the vessels to determine WHO had offended THEIR god — proof that in that period there was at least tacit understanding by the Jews that other gods existed and that their god co-operated with them.
As far as in the wider world, I would venture that because the sea influences almost all cultures in the world that Poseidon would very likely be on good terms with at least the sea god of those pantheons. He might not always be on good terms with deities like Pele of Hawai’i (who is not always on good terms with the ocean in stories I have heard from one of her devotees) or other “fire” deities, but would be on good terms with deities such as Opochtl (the Azteca god of fishing).
In each case though, I would figure out how the other deities of various pantheons relate to their own “sea god” to find out how they might relate to Poseidon.