If you have a tidal change in the morning, the fishing can be very good for flounder. The low light conditions combined with moving water is what triggers flounder to feed more aggressively.
During the middle of the day, you can still catch plenty of flounder as long as there is some moving water, but the middle of the day usually provides a slower bite than the morning and evening. The bite does improve if you have some low light conditions, so if you have some cloud cover or a light rain combined with a tidal change, you may experience a better bite during the middle of the day.
The evening is a great time to target flounder. Moving water is still important, so you need a tidal change, but an incoming tide and and the outgoing tide will trigger a good bite during the evening.
Flounder are also active at night. Some of the best places to fish for them at night is near the lights around the piers, bridges and docks. Some anglers go flounder gigging at night in the bays on the shallow flats.