It's not the end of the world if you can't make the open water swims, although they are very useful in the lead up to an event. Even if you're an experienced triathlete or swimmer.
The open water swims are there for three main reasons:
1. To get you accustomed and confident in an open water environment.
2. To get you used to the feeling of swimming in a wetsuit or your race suit.
3. To practice sighting and navigation.
You'll also benefit from practicing race starts, swim exits and getting out of your wetsuit in a hurry.
If there is no lake, ocean or river near you, the next best thing is to do is swim the open water sessions in a pool using your race-day gear.
Any open water experience you can get will be valuable, even if it's a little swim at the race venue on the day before your key event. That said, you should be fine either way if you have followed most of the training plan.