I don't think a skinsuit, or a trisuit, is any warmer than a jersy and shorts. As a matter of fact, it may even be cooler, especially if it doesn't have sleeves.

I generally race in a one piece suit. It holds my gut in.

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Tom Demerly
The Tri Shop.com

You would appear to be right. But surprisingly, a skinsuit can help even in high heat via a few tricks. I wear one and even in hot temperatures it helps me.

Here's why: for one, I am pretty tan, but my skinsuit has a white "top". So in direct sun running or biking, the reduced heat load from solar radiation on my upper body is quite noticeable wearing the white skin suit vs. my bare (and somewhat tan) chest or back. The lower part of my suit is black, but the heat on my rear and thighs doesn't affect my body core temperature nearly as much as the all white top.

But the real issue for heat and racing is the run. And my skinsuit is far cooler for me when I pass an aid station because I grab a water and press the paper cup to the fabric covering my upper chest, which soaks the upper half of the skinsuit in cold water. The suit holds the water for quite a while and cools me decently as the water evaporates (just like sweat does). When I would do similar things racing in just a speedo, the water would just run down my chest and soak my shoes. None would stay on my upper body where it was most needed.

That's why a skinsuit works better in the heat for me.

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Where would you want to swim ?

I switched to a one piece suit so I don't cook my lower back. They aren't any hotter than a sleevless jersey and shorts. They also unzip further than most jerseys. You have to try them on before you buy though because the sizing of different brands is different.