you have plenty of time, so learn it just like any other sport. start slow and build up. nothing wrong with training in a pool and if you can get a few sessions open water before the race, great, if not, don't worry about it.
one of my favorite quotes was "no one wins a tri in the swim, but you can loose one there". Meaning, if you are a really fast swimmer but stink at the rest of the events, it will not do much good, but if you swim too hard you can sap yourself for the rest of the race.
so practice and pace yourself to make it the whole distance and as you get closer, practice doing two events in sequence so your body is used to transitioning and pacing for swim/run, swim/bike, bike/run.