Common herbivores in an aquatic ecosystem such as ocean are

Molluscs are the common herbivores in an aquatic ecosystem.

In the Ocean's surface, zooplankton are the common herbivores.  A few other opportunistic grazers in include jellyfish, larval stages of some fish, barnacles, and molluscs. Larger herbivores include surgeonfish, parrotfish, green turtles, and manatees.