Well, you are probably not going to like my feedback here... and maybe I shouldn't be so honest... but here it goes...
Swimming is probably one of the hardest of the 3 disciplines to pick up. Unfortunately, you may have learned this a little too late.
You see, on the bike, you can just slow down... on the run you can walk if you need to... but on the swim it's do or die time. You can't fake it out there in the water... and poor form means that you go no where fast and fatigue sets in quickly.
The other part of the swim is that this is where you can seriously endanger yourself. You can drown out there. And if you are caught in a group and the kayaks don't see you go under and you panic... you can drown. Plain and simple. That's why I'm not going to candy coat my reply here.
I have seen too many people post on Beginner Triathlete (BT) about how they freaked in open water and how they got a panic attack. Maybe you won't, but you won't know till you get out there in the water. And by then it might be too late.
On BT we have had family members/friends post about their friends who are about to do a tri... and they are worried because their friend has not practiced enough to successfully complete the swim. And you know what we tell them... we tell them that if that person is not ready to swim the distance in open water, that person is endangering their life and advise against it.
Obviously, you are going to do the race, and hopefully you will make it out of the water. It probably won't be pretty, but you will hopefully make it out OK.
With that said, I strongly urge you to get a Swim Safe belt. That way, if you get paniced and feel your life is in danger, you can enact the flotation device and be OK. You will be DQ'd from the race, but better alive than dead. Right?
I'm sorry if my reply wasn't very positive or "you can do it", but I just felt like it needed to be laid out on the table... because swimming isn't that easy and it can mean you drowning.
"Life is not a journey to the grave with the intention of arriving safely in a pretty and well preserved body, but rather, to skid in broadside thoroughly used-up, totally worn out, and loudly proclaiming: WOW WHAT A RIDE!!!!"