Not that it may make too much difference to flytyer and fishers, but there are two major classes of swimming, schooling shrimplike crustaceans in our water: euphausids and mysids.
Euphausids (aka krill), as nicely shown in the image Big E shows, are cousins to the crustacean group that includes shrimp, crabs, and lobsters. According to Kozloff, Marine Intervertebrates of the Pacific Northwest, we have two species, Euphausia pacifica (without a rostrum) and Thysanoessa raschii (with a slender rostrum). Euphausids are filter feeders, consuming small phytoplankton. Generally, females release fertilized eggs that hatch into a series of larval stages. In my experience in the San Juans, euphausides were more common during periods of large tidal exchanges, likely because the currents carried them up to the surface.
Mysids are more closely related to amphipods (scuds) and isopods. Generally, if one is talking about "freshwater shrimp", they are talking about mysids, often Mysis relicta or Neomysis from large lakes and some deep, slow-moving rivers. We have a dozen or more mysid species; if I had to guess, that is what I think are visible is Riseform's picture from the Narrows. Female mysids, isopods, and amphipods hold their eggs in a brood pouch where the young hatch and complete development; when ready, they emerge as miniature adults. Like euphausids, mysids are filter feeders. Some species live in sandy beaches, others in eelgrass beds, others in open water. I've seen schools so thick while diving off the west side of San Juan Island, that you can't see the rocks or algae behind them. Most are completely clear, though there are some species with distinctive coloration.
Well, crap! 85 species of euphausiids (not to mention the difficult spelling), 3,600 species of amphipods and now I've got mysids to worry about? I'm hopeless.
Thanks for the information, Steve. The day I took that picture there was a huge incoming tide with these guys helpless in a swirling eddy. There were small fish (herring?) rising in an absolute feeding frenzy. I had no crustacean pattern at the time and ended up sitting on the bank watching the spectacle.