I think it's 90% not trying to impress and keeping yourself protected from the elements.
'Sweaty, harassed mess' usually comes from lack of sun protection and too much going on / too much trying to look good.
Easy and permanent access to shade, hair tied / clipped back, hat, sunglasses, a simple bikini or swimsuit, a simple cover-up, solid sliders to protect your feet from the hot sand, plenty of sun protection, some spray conditioner, lots of water, and minimal 'stuff'.
I live in a hot country and most of the people I see struggling at the beach are the ones who have tried to 'dress up' and look good. Jewellery, hair suffering in the salt and sun, complicated swimwear, trying to get a tan too quick and suffering in order to do so, wearing mad sandals that their feet still burn in the sand through... even makeup.
Think 'sleek, sensible and simple' and you'll be on the right track.