This an Egyptian symbol Ankh. It symbolized eternal life to the Egyptians. This symbol was found in many writings and carried as amulets by the people. The afterlife played a big part in Egyptian culture
"The word ‘Amen’ is from Amon, the father god of Egypt, and was an ancient Egyptian salutation to the supreme power of the Universe." - Manly P. Hall
Poseidon is one of the twelve Olympian deities of the pantheon in Greek mythology. His main domain is the ocean, and he is called the "God of the Sea"