What makes one into a god? Well, apparently sometimes being large enough is all it takes. At least that was the case with the Sea God that our heroes encountered during their sea trip to Elfhelm. In Japanese, his name is pretty straightforward: 海神.
It literally means "sea god", with the first kanji meaning sea (海, commonly pronounced UMI) and the second meaning god (神, commonly pronounced KAMI). When combined, the resulting word is pronounced かいじん (KAIJIN). We get confirmation of that at the beginning of episode 312, with furigana on the word when Isidro says it.
Related: Check out our article about Dark Horse's mishandling of the pirate captain.