Tips for swim leg?

It's most likely a balance issue.  I struggled a lot until I watched this BT video and then I had a better understanding what I needed to do.  For the balance part skip ahead to the 32 minute mark where Terry starts the balance discussion.  Dropping the hand and relaxing my head lifted my legs and I was able to drop 15 sec/100 and maintain the pace.  There's a lot if of good info provided, work on the balance then you can work on the other aspects such as the pull, rotation, etc.

FWIW, I basically flutter kick unless its the last 100 or of a swim leg or trying to pass someone. 

I would actually caution against using the pull buoy 100% at your level of super-beginner swimming. 

THe OP clearly says he can already do like 1.1k continuously with the PB, but can't maintain 150-200m without. And furthermore, the proof is in the pudding - he clearly wasn't able to finish the swim comfortably on race day, despite his ability to cover similar distances with a PB.

I think you have to at a minimum, be comfortable swimming sans buoy for longer distances, at relaxed paces (and preferably better than that) before the PB isn't compensating for some serious form errors. 

It is true that there are some high level swim coaches (whom I agree with) who say 'use the pull buoy all you want! It's like the wetsuit!' but if you look at who they're training, they're NEVER raw beginners, and more often than not, FOP triathlon swimmers looking to further increase volume. For sure, you can ingrain some bad leg-drop habits if you're completely reliant on a PB to keep your legs from dropping a lot, even with the wetsuit assist.

I'll jump in because I was a complete non-swimmer before 2008 - it takes a lot of work and patience - so good for you on getting to the pool and keeping on with trying.

there are lots of good things here already offered in the thread but here are some of my thoughts:

Although i didn't follow Total Immersion to the letter, I did look at the book and a few videos and there are some really excellent body position drills. Like 'pressing your buoy' where you'll learn to find how to get your legs up. Even my swim coach who was a pro triathlete would draw on TI drills for our coached swim sessions. No reason to discount the whole thing, as above. 

You also mentioned doing a try-a-tri and not wanting to but it is a great place to start! I did marathons before getting into tri so the super short distance didn't seem worth it but for getting comfort in the open water and experience of a group start they are really great. So definitely do one if you're worried about the distance. You'll still feel really accomplished when you make it through the swim

I agree with the others - just book a couple of lessons if you can - no need to do it all the time but it might be worth a couple of hours PTO or making some adjustments to fit it in a couple of times.  

I also had really good luck with the three month beginner swim program on here - Beginner swim program, month 1 - i used it once i had my lessons and felt okay to go on my own. 

ETA: link to swim program

In college I was entering triathlon as mainly a runner (who had become injured). Strong legs, weak upper body (relatively speaking). And I'd learned to swim as a youngster using the 6-beat kick. Great, until your strong legs are trying to do most of your propulsion, and sucking up most of your available oxygen. Even when I was making sure to exhale completely while my face was in the water, I was sucking wind at the pool edge after 100m.

Here's what I had to do -- for 6 weeks I swam ONLY with a pull buoy. I did that in order to UNlearn the 6-beat kick, and then I started all over again with a 2-beat kick. The difference was dramatic and instantaneous.

Just a datapoint.