It’s interesting that, while Poseidon is one of the best-known gods of the Greek pantheon, I haven’t heard all that much about Norse sea gods. Kind of odd, as the Vikings were more famous as seafarers than were the Greeks. Anyway, there were actually several Norse sea gods. The most famous was probably Aegir, who was considered disagreeable and bad-tempered, yet also threw parties and brewed the best ale known to the Norse deities.
Aegir is sometimes depicted as a giant, and is quite possibly of Jotun ancestry. His father is the giant Fornjótr, who once ruled Finland, and his brothers the fire god Logi and wind god Kari. Sometimes Aegir is shown as simply a thin old man with hair resembling sea form and a crown of seaweed. His wife Ran is said to be quite ugly, and to use a magic net to capture and drown sailors.
The dead sailors do get to feast in Aegir’s hall during their afterlife, however, so I guess it isn’t all bad. This hall is located underwater just off the island of Hlesey, now part of Denmark and known as Læsø in Danish, and has a floor covered in gold to provide light. Aegir and Ran have nine daughters, symbolizing the waves of the ocean, whose names vary from one source to another.
It is thought that these wave deities are the nine mothers of Heimdall, with Odin as the father.
In one myth, Aegir and his daughters brew ale for the gods in a giant cauldron that Thor and Tyr obtain from the giant Hymir, who is sometimes identified as Tyr’s father. In another tale, Loki borrows Ran’s net to capture the dwarf Andvari.
Another Norse sea god is Njord, one of the Vanir, whom you may remember from his brief marriage to the snow goddess Skadi. He doesn’t seem to have been featured in many known myths, but he might well have been more popular, as he wasn’t as openly hostile as Aegir and Ran. He was known for controlling the wind, and worshipped as the patron of fishermen. If someone wanted to get a good catch, they’d pray to Njord. According to his Wikipedia entry, he was apparently thanked for fish even after Scandinavia became Christian, perhaps even as late as the eighteenth century.