Recall that the difference between simmering and poaching is primarily one of temperature. Simmered foods are cooked at about 185°–200°F (85°–94°C), while poached foods are cooked at 160°–185°F (71°–85°C). The lower temperature of poaching better preserves the texture of delicate fish items. Simmering is used primarily for shellfish such as lobster, while fin fish are rarely simmered.
As explained in, the liquid typically used for poaching fish is called court bouillon. As defined there, court bouillon consists of water containing seasoning, herbs, and usually an acid. Refer to the recipes.