I thought that I knew what the collective nouns for marine mammals were but then a friend told me that a group of seals could be called a 'pod', a term I thought reserved for some cetaceans, but friend was correct.
So here is what I think the collective nouns are now:
Pod is the terms used for whales (usually a small group) and seals. I believe it is most correctly used for orcas and pilot whales rather than other species. It is sometimes also used for other animals and in particular hippopotamuses and whiting (a species of fish).
Herd can be used for sperm whales and others.
School can also be used for a group of whales as can gam (which also indicates a social gathering of whalers at sea).
Grind is another collective noun for cetaceans, apparently specific to northern bottlenose whales and the term used for a hunt in the Faroe Islands.
Porpoises (which I suspect includes dolphins in this respect) have four collective nouns associated with them: school, herd, pod and gam.
Seals have three: herd, pod or rockery (although hitherto I thought rockery applied to the haul-out site rather than the collection of seas).
Anyone know better?
Some other collective nouns:
Parrots: a company.
Otters: a bevy.
Nightingales: a watch
Monkeys: a cartload
Hummingbirds and finches: a charm
Badgers: a cete
Kittens: a kindle.
Gorillas: a whoop
Sources: various dictionaries and Stephen Palin's beautifully illustrated 'A Murmuration of Starlings - The collective nouns of animal and birds.' Published by Merlin Unwin Books.