At this time of year, the water in the shallows will have a very large temperature swing due to solar heating.
However, the fish are only experiencing the temperature swings that they choose to experience. The warm surface water is less dense, and will remain at the surface (absent severe wind mixing). The deeper, cooler water is very stable at this time of the year, and should be warming at a very slow rate.
The fish that want to take a dip in the hot tub, can go swim in the shallows during the day. The fish that like oxygenated cold water can hang out in their preferred spots.
