Fish
From Halopedia, the Halo wiki
These are species of fish native to Earth around Voi. They are possibly Bass or Tiger Fish.
These fish can be found on the Halo 3 level Sierra 117, at the chapter Charlie Foxtrot, where the Pelicans are attacked by Covenant Loyalist Banshees. If a player jumps in the water, that person may walk around and see them. They will appear as small yellow dots on your radar screen.
It is possible to kill them; if you do, they float to the surface and then drift to shore before eventually respawning. Throwing a grenade in the water will result in the whole "school" of them dying and floating to the surface.
They are obviously different from the type of fish seen at High Ground, as they are much larger, differently colored, and lack the odd snout that the High Ground species appears to have.