During summer, though the surrounding is hot, the water in the lake will not reach very high temperature mainly due to its high specific heat capacity. In general, cool water sinks as it is denser and warm water rises as it is less dense, the bottom of the lake will always be lower temperature compared to near the surface. The bottom of the lake may not necessary be 4 degree Celsius.
In a vacuum room, where is no air resistance, the bowling ball will have the same acceleration as the feather. That means to say that both will have the same speed at any time and reach the floor at the same time.