An IPOD. Fishing takes a lot of time and patience. Best to have something to occupy your time while you wait. Just make sure to keep the cords neatly pressed to your body and away from all fishing materials.
LOL! Well I consider it an essential because I am not a very patient person. But as for fishing essentials lots of good bait, strong reels, and a cool fishing hat always work. Some food also. Peanut butter crackers are my food of choice on a fishing trip.