I’m researching sea lore for a pirate themed thing I’m making and I’m interested in mythology aspects to incorporate. I know of the classic Kraken of course but what other mythological creatures “roamed” around that same time period. I don’t really mind which mythology origin it stems from (Norse, Greek, etc.) I just want to know what kind of beast were right for that period that I could further research.
There is no sort of generic folklore dealing with the ocean: each culture that deals with the open water has its own beliefs and stories, so ultimately, yours is an open-ended question that could be addressed by a full range of specialists dealing with every continent (and island!) and its people.
One of the more ubiquitous beliefs is that the oceans are peopled by either half-human-half fish "merfolk" or that these people assume the forms of seals and that they can shed their seal skins to take on the appearance of normal humans. I published on this topic from the Cornish point of view in 2015. Since that time, I have changed the article to serve as a chapter in my forthcoming book, The Folklore of Cornwall: The Oral History of a Celtic Nation. The article, however, should lead you to other sources that may be useful to you - and it can give you an overview of beliefs in mermaids.