The plants and the fish can die. On the short term, the oxygene content of the water is the problem. The oxygen content decreases with increasing water temperature. So beyond a certain temperature, the oxygene levels can go below the levels required for survival, the animals and plants suffocate. Especially in lakes it can happen, that in these weather conditions basically all fishes can die at the same time. Agitating the water helps to maintain higher oxygene levels.