Some people say:

• a bodysuit pattern = a swimming costume pattern.
• I'm going to take the bodysuit pattern that suits me and make myself a swimming costume ...

Who hasn't said so or thought about doing so?

Well NO, it's not the same thing.

Why?

For 2 simple reasons:

• The rate of reduction

The 2 garments are not designed with the same reduction rate (see the article What is stretch fabric?). A bodysuit is designed with a reduction rate of around 8%, whereas a swimming costume is designed with a reduction rate of 12-15%.

• The textiles used are not the same

To make a bodysuit, we generally use a fabric that has less bounce, is softer and lighter (lower weight per m2) than those used for swimwear.

Bath textiles generally have more bounce. 

If you use the same pattern to make a bodysuit and a swimming costume, the result will not be the same.

I would also point out that the comfort required is not the same.

Mind you, I'm not saying that a swimming costume isn't comfortable 😊