Here are a couple of introductory, non-technical pages on ostracods:

http://www.ucmp.berkeley.edu/arthropoda/crustacea/maxillopoda/ostracoda.html

http://www.arctic.uoguelph.ca/cpl/organisms/inverts/marine_inverts/ostracods
.htm

Though vernacular names have no official standing, the name "water flea", in
my experience, most often refers to free-swimming cladoceran crustaceans
like Daphnia sp.  The common name for ostracods that I am most familiar with
is "seed shrimp".

Regards,
Paul M.