Does traditional astrology have a ruler for anything that wasn't known before the twentieth century?
The mammalian diving reflex was identified in the 1930s. Ancient astrologers wouldn't have known of it.
They would have known of aquatic mammals, though: seals, whales, dolphins, etc. If there's a planetary ruler for those creatures, that would probably be it.
Neptune is also a good fit for aquatic mammals too, since they have more 5D energy than the rest of animals, even us humans and other primates.For large aquatic mammals, I would guess Jupiter as the ruler since it rules Pisces the sign associated with fish.