We know that if we keep plant cells in water, they don't burst because of a cell wall. But the cells of aquatic animals lack cell walls but they still survive. Why?
Comparative physiology of body fluid regulation in vertebrates
Cells are not passive components that always return to osmotic equilibrium. Through hormonal, cell signaling, and the number of pores for ion and water exchange marine animals can maintain osmoregulation in various aquatic environments. In land mammals osmoregulation is more passive since it is controlled by the endocrine system.