Sea critters are special because your capital may not be on the coast. Which means that if it was not on the coast you would not have any sea critter myths. To fix that I made it so that you can choose where you want the myths to go by making a "Myth of the Sea" which is the same restriction as Palace for the other myth buildings. The "Song of the Sea" stops you being able to build any myth buildings in other cities.
At the start of the game you only get "Tales of ...". When you capture a sea creature you get

,

and "Tales of..." back at the nearest city. The "Tales of..." can then be used for a Myth building
or +1

and +1

. (
@AIAndy for some reason the science outcome is no longer showing on the hover over.) You can't butcher them because you already have.
As time goes by you start to be able to capture sea animals, the smaller fish first. When you do you don't get any

or

just the "Subdued...". Depending on your latitude and what Admin buildings you have you can build "Fish Pens" and later "Coastal Fisheries".
At "Marine Biology" iirc, you can start to capture the larger sea animals. "Marine Biology" opens up a number of sea creature related buildings.
Edit "Tales of Narwal" and "Tales of Dugong" can build myth buildings n any city.