Morning. Hands down. The earlier the better. The air is clear and, most important, smooth. As the day wears on and thermals develop, it just gets rougher and rougher.
If you want to see deep into the water, use a polarizer. In fact, if you have enough stop, use a polarizer anyway. Coastal atmosphere is notoriously hazy. Make sure they allow you to remove the shooting door off the helicopter. You can't shoot through the plastic windows with a pola.
Minimum shutter speed is 1/1000th.
Have fun. Helicopters are magic carpets.