Even if, by some chance, you don't get to practice in open water, don't worry. I did an ocean swim two years ago having not done ANY ocean swimming for years (I'd been to the ocean, but that was for just wading/getting cooled off, not going out past the breakers). After some initial anxiety getting through the breakers, I was fine. At any official swim event, you'll have a lot of people on kayaks, in boats, maybe also on surfboards who will be looking out for you. In my ocean swim, I met an "angel" on a surfboard who talked me from the end of the breakers to the first mark I had to round. She was wonderful! In all of the swims I've been in, the volunteers patrolling the water have been GREAT--really helpful and friendly.
Yes, if you can get a chance to swim in open water, go for it--but I have done open water swims with little or no open water practice in the weeks/months preceding, and I've been okay. It might have helped that a lot of my early learning to swim wasn't in a pool but in a bay. I think that makes me less prone to panic if I encounter things like seaweed, chop, etc. In lots of ways, it's more fun than a pool--more freeing, close to nature, often in beautiful surroundings (not that you can see them too well concentrating on swimming, but can always enjoy before/after), more interesting than just going back and forth.
Resist the urge to sprint out.from the start, because often that's what unsettles people--I've done that and have to stop and catch my breath and calm down. Once I did this, I was fine, but it's better if you don't worry about the people sprinting out ahead. Some of them may be going out too fast and not able to hold the pace they started with, and you'll likely catch them if you keep your own pace.
Good luck!