"I waded about fishing for the little shrimp-like creatures that lived there"
Gnathophausia zoea
GNU Free Documentation LicenseGnathophausia zoea - Credit: Alexander Semenov
These could well be Mysidacea, a group of small, shrimp-like creatures. They are also referred to as opossum shrimps, but are only distantly related to true shrimps.

Mysidacea can be found both in salt and freshwater, including in the Great Lakes of North America.