There is no Poseidon Kingdom. There's Atlantica, the Little Mermaid's world, but like all worlds in Kingdom Hearts, it's opened just by making regular progress in the game. There are no secret worlds in KH, just secret endings.
Swim
You pass the grove of temples and hades kingdom and you get to Poseidons kingdom
The sea, particularly the Mediterranean.
You need to get the starfish from the museum of the gods. The starfish is on the Poseidon statue's face. Then you can go to the offering altar in Poseidon's temple to open the door to get to Poseidon's beach.
Dive below
With Aphrodite's magic mirror.
Yes
his responsibilties were to keep the sea at peace and control the kingdom below it ;)
No, apollo did
I wish it was but unfortunately NO
It is said to be Neptune's kingdom. Although in Greek mythology Neptune is Poseidon, I believe his kingdom is called something else though. so the answer is no but technically it's yes!
Poseidon told Odysseus that he would never again set foot on the land of his kingdom, Ithaca, after he boasted to Poseidon that he defeated Troy without the help of the gods.