Which cooking method is primarily used for cooking fish to retain moisture?

Poaching is an ideal cooking method for fish when the goal is to retain moisture. This technique involves cooking the fish gently in water or a flavored liquid at a low temperature, typically below boiling. The gentle heat allows the fish to cook evenly without the risk of drying out, which is particularly important as fish is often delicate and can easily become overcooked. Additionally, poaching can enhance the flavor of the fish, as it absorbs the subtle tastes of the poaching liquid.

In contrast, methods like baking and grilling expose the fish to higher heat levels, which can lead to moisture loss and can create a firmer texture, sometimes resulting in dryness if not monitored carefully. Searing, while effective for developing flavor through caramelization, also does not prioritize moisture retention and generally involves higher heat to create a crust on the outside of the fish, which can lead to moisture escaping from the interior. Therefore, for preserving moisture and achieving tenderness, poaching is the best-suited method among the choices provided.