You seem to overestimate our knowledge of the sea. There's a great deal we have never witnessed, but have educated guesses on how it works. Things like where / how many sharks birth. For example, we know big preggered whale sharks have been seen migrating to to the same area year after year, but we have never witnessed the actual birth or baby whale sharks afaik.
Hell, I don't think we've even witnessed how penguins at sea sleep. It's assumed that they rest by turning off parts of their brains in cycles like many other water life, but these are educated guesses.
- IANA marine biologist