At the outset, I need to point out that I have only occasionally written anything that could be referred to as maritime folklore. With the limits of my research understood, I have found that any one culture can have several dramatically different views of sea. People who live near open water can have beliefs and stories without every going in a boat. And then those who make their living at sea can have their own, occupational traditions that can be similar and yet sometimes very different from hose on land. Then there are also those who live off the sea at the coast (gathering kelp, fishing from land, scouring tidal pools, scavenging flotsam, etc.)

We tend to look at maritime folklore as a large stew of related beliefs, but it is important to remember that the stories and beliefs of a sailor could be very different from those of a farmer whose fields look out over the ocean. I have noticed that in a big way when dealing with The Folklore of Cornwall - the name of my most recent book.

I have a chapter on mermaids in that book. It draws heavily on an article which was dramatically improved for the book, but which I offer here with the link provided because it is free (and the book is too expensive!!!). The thing about mermaids is that they seem to be the most obvious of sailor's lore, but they are mostly found in coastal - if not tidal! - traditions and legends. They belong to that liminal space separating the secure terra firma from the unfathomable ocean. Until very recently, they were very much a matter of belief and stories for both the people of the land and the people of the sea, although sailors would look on them slightly differently: in Britain and Northern Ireland they occupied a role not unlike the fairies on land, in that they live in communities and families, and they were extremely dangerous but if treated with courtesy, they could be beneficial. That doesn't say as much as I could, so ask questions and I will response as much as I can.

The question of the souls of the sea dead has been a long-held interest of mine, largely because there is an Icelandic saga that describes a well-known legend about the sea dead in a way that reveals how Christianity affected the narrative. For this, I will draw on an excerpt from the Introduction to Folklore that I used over the decades when I taught folklore classes:

There is at least one example of the successful incorporation of the idea of good and evil into the traditions of the peasantry. This occurs in the legend of the land dead and the sea dead. In this story, a young man walking along the coast at night insults the ghosts of the sea who begin to chase him. The frightened fellow flees through a churchyard and, hurrying past the graves, he cries out, “Up, up, every Christian soul, and save me!” He hears a tremendous noise behind him as he runs to his home. The following morning, the townspeople find jellyfish, sea tangle, and boards from coffins strewn about the churchyard. They conclude that the ghosts of the churchyard must have fought with the sea dead to protect the young man.

Christiansen classifies this story as Migratory Legend 4065, “Ghosts from the Land Fight Ghosts from the Sea.” The story serves as an example of peasants applying the Christian dichotomy between good and evil: the dead who are buried in consecrated church ground are helpful and good, while those who were lost at sea without burial rites are outside Christianity and are, therefore, evil. An early variant of the legend illustrates that this was not the original point of view.

The thirteenth-century Icelandic Eyrbyggia Saga includes an episode that is strongly reminiscent of Legend 4065. A series of illnesses left several people dead and buried, but their animated corpses began to haunt a certain farm. Shortly after this, a ship from the same farm was lost at sea and five sailors were never found. The household had a funeral feast for the lost men. No sooner were the people seated then the dead sailors entered the room, leaving seaweed and puddles of seawater wherever they went. The corpses warmed themselves by the fire. They returned each night, even after the funeral feasts had ended, but now even more dead people arrived, these covered with dirt, which they shook off and threw at the dead from the sea. The two groups, those buried on land and those lost at sea, met each night and quarreled until the owner of the household charged them with trespassing, at which the corpses left for good.

This early variant of Legend 4065 lacks a Christian point of view. Both groups of dead are equally troublesome, and neither has assumed a good or evil role although they are clearly antagonistic to one another, anticipating the later legend. Since recent variants of the legend exhibit the Christian dichotomy of the spiritual world, it seems likely that peasants modified Legend 4065 sometime after conversion. Although the legend is an example of the successful integration of the concept of good and evil, it is the exception and not the rule.

With this story, we see a fundamental belief that those who died at sea were naturally antagonistic when confronted with those who had received a burial in the earth. It is part of what sets sailors - probably in their own minds - apart from those who tiled the earth. Every sailor knew that there was a good chance that he would at some point join the realm of the sea dead, apart from the community shared by the living and the dead on land. I can't speak to the idea of seagulls being the souls of dead sailors. I do see that Stith Thompson in his enormous "Motif-Index of Folk-Literature (1955) cites an example of this belief: F423.3.9 "Reverent as sea-gull," citing only one source, that being A. H. Krappe's 1927 study, "Balor with the Evil Eye: Studies in Celtic and French Literature." It has been four decades since I read that, and I sadly I don't have it on my shelf, so I'll have to leave it at that, evidence of the profound limits of what I can say in response to your inquiry.

That said, feel free to ask questions. I'm sure I have failed to give you much here.