By nature, a pool party is casual and fun. It usually means bathing suits, loose cover-ups, sandals and lots and lots of sun-kissed skin -- all of which is supposed to be sexy.

But Rumer Willis, the eldest spawn of Demi Moore and Bruce Willis, missed that whole "be sexy" memo when she appeared at the Marquee Pool Party in Las Vegas to celebrate the opening of the Marquee Dayclub on April 6.

While her flat, toned abs are enviable, WTF is up without those mandals? A skinny strap, jeweled thong or even a pair of metallic Havianas would have been a better choice than ugly, chunky man sandals. (She should also ditch the wine-hued pedicure in favor of a spring color like pastel blue or mint green, but that's a secondary concern.)

Pacific Coast News

Rumer's ombre extensions obscure her triangle bikini top. But the waves aren't effortless or beachy -- they're just ... dirty. She would have looked cuter with a messy top knot piled high on her head, which would call attention to the bangin' body below.

And that sparkly mini isn't beach-ready. A sarong or a wrap would have have been appropriate and highlighted a figure that deserves the attention.

Pacific Coast News

The too-long cover-up isn't designed for pool lounging either -- it should've cut off at mid-thigh, not at the ankle.

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Notice her posture? She doesn't look comfortable with all this either. The remedy is a pair of toe floss sandals, a sarong and a top-knot bun. Those elements would make this morph from Debbie Downer to dazzling.

She doesn't even seem happy perched on Jayson Blair's arm. At least he looks somewhat resort ready.

Pacific Coast News