Lynn - as someone who has done a lot of ocean swimming and a little bit of fresh water swimming, I can tell you that you're in for a treat. Salt water is more buoyant than fresh, so it's easier to swim in. I wouldn't worry about sharks - they really are not interested in humans as a food source. The majority of bits occur when sharks get confused by what they think is fish and turns out to be a leg. They generally avoid people.
The main thing you may have to deal with in salt water versus fresh water is waves. You should become familar with the way to make your way out to the buoy through the waves without fighting them which will make you tired. It's called dolphining. When you are heading out to the buoy, you wade in (or run in) until you are about thigh deep, at which point you can start swimming. As waves come in, dive under them so that your hands touch the bottom. Bring your feet up to you hands (you're now in a crouching position), and then push off with your feet while bringing your hands into a prayer-like position to help you easily break the surface of the water. Get a breath and continue the dolphining as necessary. Usually once it's too deep to dolphin the waves are no longer breaking on you and you should be good to go.
Remember, too, that when you first get in, if there are waves to stand sideways to them to allow them to break around you. Allowing them to break on your full front or back takes more energy to remain standing.
It might help to find a tri group in the area and see if anyone will meet up with you for an hour or so to walk you through the finer points of what I described above. They can also give you insight into wet suits, typical water conditions, etc.
One more thing - on race morning, do a swim warm up and take a good look at the bottom to note where there may be rocks, as well as where the water gets deep or maybe a sand bar formed a bit offshore and it suddenly gets shallow. Talk to the lifeguards the days leading up to your race to find out what the conditions are like.
I hope you decide to do the race. The ocean swim is a fun and different experience.
"God gave you your body as a gift, so you should take care of it." - My Mom