Having a fear of the open water is quite common. But lets be realistic, unless you are swimming in the beaches of South africa, Australia or the US west coast not much will happen to you. So, that being said if I were you I would attack the mental problem head on, and again there is no shame in what you have said about your possible phobia. I work with a girl who wont swim in lakes for fear of the fish.......
If it were me I would concentrate on more open water swimming/practicing, but take it slow at first, maybe with a friend. Preferably a fairly clean lake and one not filled with algae, weeds, etc....where you can see the bottom like Mirror Lake in Lake Placid, NY where the Ironman was. Keep the depth shallow and get away from the pool if at all possible. I am a SCUBA Instructor and I know half the battle with our students was adapting to a differant environment, but it can be done with practice,patience and determination.
The swim portion of any triathlon is the shortest distance, but the hardest mental barroier to overcome for many. You are not alone please believe me.
If you live anywhere near Raleigh, NC let me know and Ill offer my help(free..just a beer or two mate)and Ill help you conquer your demons. It can and will be done.
cheers, Mike