@Background Pony #E3AE
It depends on material, angle and light. There are swimsuits who can have this effect. Here are a few examples:
1,2,3

I agree that AI is a little excessive with the effect, but it can happen.

Edit:
And AI can do the other way too: Link

Beautiful. But I still can’t help but point out the obvious AI pains. The belly button and sticky underboob swimsuit. Sometimes I try something small on Civitai and I just can’t get it to make the swimsuit not stick to the underside of the breasts, creating their outline. I don’t think that’s what’s happening in real life. Same with the belly button - there’d have to be a vacuum in there to look like that. Normally belly buttons aren’t visible under swimwear. But AI just keeps insisting.