Kurayamatsumi is a minor deity of mountains, ravines, and gorges. As such, he is part of the Yama-no-kami (山の神), a group of earth or mountain gods, goddesses, or spirits. His name has the alternate reading of Takaokami (高靇), which can mean “high rain-dragon.”
Kuramitsuha is the sister of Kurayamatsumi, but because of the conflicting order of events between the Nihon-Shoki and Kojiki, she is sometimes considered the same deity as Mizuhanome (or Mitsuhanome) which can also be read as “Dark Water Snake” or “Valley Water Snake” which leaves the possibility that she was also a dragon deity.
Ōwatatsumi is a guardian water deity who rules over the sea, but he has two alter-egos. His most popular name is Ryūjin (龍神; Dragon God), which is believed to be an alternative identity representing one of the eight dragon kings imported from the Lotus Sutra of Indian Mythology.
But Ōwatatsumi is also sometimes considered Watatsumi Sanjin (綿津見三神, “Three Watatsumi gods”) which are the three evil dragon gods who rule the upper, middle, and lower levels of the sea. They were created by Izanagi when he washed himself after leaving Yomi, the underworld, to try and save Izanami from death.
Although it is somewhat confusing to have three (or five?) deities connected under one name, I think this can be interpreted as the capricious nature of the sea, which can guide and aid you one minute, and then drown you the next.
Ōwatatsumi lives at the bottom of the sea in a palace made of coral (or fish scales, depending on the story) with his daughter, Toyotama-hime (豊玉姫; “Abundant Pearl Princess”). He is the power of the ocean, thunder, and rain, and, according to legend, can turn into a human and uses the jewels of the sea to control the tides. Snakes, jellyfish, sea turtles, and fish are his messengers and servants.
In addition, Ryūjin is said to live in Lake Biwa (north-east of Kyoto) and is often associated with the freshwater deity Suijin (水神; “Water God”), whose name is also used to refer to other mythological water creatures including snakes, dragons, kappa, and more.
There are many myths and stories associated with Ōwatatsumi/ Ryūjin, but perhaps the most notable stories are those surrounding his daughter’s marriage to Hoori. Long story short, Hoori and Toyotama had a fateful encounter, after which Ryūjin recognized Hoori as a descendant of the Gods and arranged their marriage.
Their union resulted in a son, Ugayafukiaezu, who ended up marrying Toyotama’s younger sister, Yamayori. She eventually gave birth to the First Emperor of Japan, Jimmu. This story established the relationship between the Imperial line of Japanese emperors and the sea god, Ryūjin, a lineage which is still in place today.
Kuraokami, the Dragon God of Rain and Snow, is worshipped all across Japan in temples alongside Suijin. Often he is the go-to deity when there are droughts or too much rain (although other local water dragons are also an option), but he is also prayed to for more snow. In some versions of the myths surrounding Kuraokami, Watatsumi, and Mizuhanome, Izanami created them to tame Kagutsuchi’s fire if he ever got out of control.