How do saltwater fish gain back the water they lose by osmosis?

Salt water fish do not gain back water lost through osmosis. Rather, they actively drink in seawater and actively excrete excess salt through their gills. They have special cells in their gills called chloride cells that use energy to pump salt out of the body. This process is called active transport.

In sum, saltwater fish do not gain back water, but rather drink seawater to balance the water lost by osmosis. They maintain their water and salt balance through active transport of salt out of the body.