Dictionary of Ichthyology  

lagoon = 1) a shallow pond or elongate channel separated from the open ocean by a sand bar or reef, or by a narrow outlet, with little or no freshwater input.

Agua Hedionda Lagoon, Carlsbad, California from Wikimedia Commons: http://commons.wikimedia.org/wiki/File:Agua_Hedionda_Lagoon_1.jpg

Canadian Museum of Nature
www.briancoad.com