I had the pleasure of photographer Doug Seifert's company a while back when we dived together for an adventurous
month on MV Golden Dawn in Papua New Guinea. He showed me a splendid article he had published on
sharks. In the text, he used the collective noun for sharks, something I did not know, and you may not have heard
of either. But it is a beauty - - a "shiver" of sharks.
The authority on collective nouns is James Lipton, whose book An Exaltation of Larks was first published in
  1968 - - it is still in print today, and it is a fascinating read. For example, if whaling ships meet, they are a "gam"
  of whalers, but the word is also used to denote a playful group of whales. "Pod" is correct for a small group of
  whales (never use a "school" of whales) or for a "pod" of seals. Then we have a "bale" of turtles, a "smack" of jellyfish,
  an "army" of herring, a "scuttle" of crabs, and a "kettle" of fish - - or perhaps a fine kettle of fish when diving
  Papua New Guinea!
I was charmed by Lipton's account of his hunt for collective nouns, and his invitation to contribute where no
  present collective noun exists. Then I realized there is no collective noun for divers. Doug and I thought of a few
  possibilities, then I asked diving friends for their suggestions - - they came up with "gurgle," "compression," "free
  flow," "bubble" and "float" of divers. Personally, I think "asylum" of divers is totally appropriate - - most landlubbers
  think we are crazy. Now I am putting it to you. If you have a bright idea, or if any of the ones above are your
  favorite, please let me know. The diving world depends on us!
I am sure you are familiar with "a swarm of bees," but perhaps not so sure of the difference between swarm
  and school when applied to fishes. In a school of fish, the fish align themselves in roughly parallel formation, with
  individuals slightly behind the fish in front. In a swarm of fish, individuals are randomly positioned. Strangely,
  research has shown that schools are actually led from the rear (as are good armies). A "shoal" means the same, I
  believe, as a "school," but is usually in shallow water, thus looking like an area of shoaling water. Some fish school
  only when it is a survival advantage for them to do so, and are termed "facultative" schoolers. Other fish school
  compulsively - - even if only two fish are together, they will line up with one slightly behind - - and these are
  known as "obligatory" schoolers. At other times, fish may group together in "aggregations" rather than schools or
  swarms; hammerheads and mating groupers come to mind.
Because we are all about words today, I will finish with the difference between fish and fishes. "Fish" refers to
  one fish, or to more than one fish when all the fish are of the same species. "Fishes" is the term used when more
  than one fish of different species are referred to. At least this is how all the ichthyologists that I have ever met
  refer to them, and how the Oxford English Dictionary (OED) used to refer to them. But it has all changed.
I know this because I was asked to write a chapter in a book to be called the The World of Fish. I complained
  and said it should be called The World of Fishes. They insisted, so I wrote to the OED, expecting confirmation. Alas,
  OED told me they try not to be proscriptive (say how a word should be used); instead, they are descriptive and
  say how a word is generally being used today. According to them, people just don't use "fishes" anymore; they use
  "fish" for everything. Unless you belong to PETA, in which case you use "sea kitten," but that is another story. In
  other words, they have given in to all the phallcephalites using the word incorrectly! Don't let it be you!
Bob Halstead, considered the father of Papua New Guinea liveaboard diving, is a well-known diving curmudgeon and a frequent contributor
  to Undercurrent's blog. Read more of his commentaries at www.undercurrent.org/blog.