What Shorts to buy.

I would check the race rules. To my knowledge, buoyancy garments aren't allowed in IM events that are not wetsuit-legal. At any rate, I would not plan on wearing them, or depending on any buoyancy aids to finish the swim. If the water IS cold enough to make the race wetsuit legal, then you'd be better off with a wetsuit.

Your best bet is additional swim training and a good pair of tri shorts. 2XU and Zoot are reputable brands; there are plenty of others but I have had the best luck with those two brands as far as fit and comfort.

It's true. You will not be able to use buoyancy shorts if the swim is not wetsuit legal. It is an unfair advantage. I mean you can wear them, but you will not be considered for age group awards.

If you are looking for real quality triathlon clothing (including shorts), then you are going to want to check out De Soto Triathlon Company (https://www.desotosport.com/) I have a pair of De Soto shorts from six years ago that are still going strong. There are some other really good companies making triathlon apparel, but De Soto may be the only company that creates all of their clothing in the US.

Disclaimer! I am an Ambassador for De Soto Sport this year, but I have been using and loving their gear for many years now, which is why I advocate for them. Anyway, you should check them out. If you find something you like, PM me and I can send you a discount code.

Best of luck in your race!