Wear the wetsuit. You'll be more relaxed and faster in the water.

There are a lot of races in my area that are borderline wetsuit legal and that is why I own a sleeveless wetsuit.

You could also consider a "simsuit" or Neoprene swim short.
Most of the advantages of a wetsuit, non of the heat gain in warm water.
Also great for pool workouts.

Ia that 78 degrees the water or the air temp. Water temp ?
Fheck, I'd have heat stroke from swimming hard in 25.5.C water in a wet suit.

It's really the water temp that matters with the wetsuit, because if it's just warm air temperatures, as soon as you get out of the water, you can unzip and roll it down to your waist until you get to your sport in the TZ... In general with the age group cut offs for wetsuit swims in terms of water temp, if the water is right on the threshold and the air temp as well, you're going to be hot bordering on uncomfortable (might still be worth it for a shorter swim, but on a longer swim you'll just poach in the wetsuit). For a shorter swim near the threshold on air-temp/water temp, you should be fine, but the longer the time you will spend in the water, the more you'll poach in your wetsuit. So in the end it comes down to comfort, if you're more uncomfortable swimming without the wetsuit than you are overheating, keep it on, if you're the opposite, ditch the wetsuit, you might lose a min or two in the swim, but will make some of that up in transition without having to remove the wet suit, and will make the rest up late in the race, when others start to feel the effects of overheating...

Good points. With the race starting at 8:30am, the sun is going to be well overhead at that point if not cloudy and the air temp is going to be rising quickly. This isn't some sunrise swim start. Think I'll be going with a swimskin since my only wetsuit is full sleeve. The bike gets intense pretty fast as soon as we hit singletrack and anyone overheated from the swim is going to start cooking right off the bat. I am also terrible at taking my wetsuit off, I'll save at least 30 seconds by not using it.

There aren't many feelings worse than overheating in a wetsuit.
It's extremely uncomfortable and borderline dangerous. Also, you will almost never be able to recover during the race.
If it's close, I wear a swimskin.

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"Good genes are not a requirement, just the obsession to beat ones brains out daily"...the Griz