Note that in addition to reading 海神 as watatsumi, wata no kami, or unagami in native Japanese kun'yomi pronunciation, it is also read kaijin or kaishin in Sino-Japanese on'yomi (from Chinese haishen 海神 "sea god").
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Compare the Japanese rain god Kuraokami that was similarly described as a giant snake or a dragon.
The blood that dripped off Izanagi's sword created a number of deities, including the sea god Watatsumi and rain god Kuraokami.