Re: Beach make up
I like to use a tinted moisturizer/bb cream and a powder foundation.
The powder helps to keep away shine (and if it has an spf helps protect your skin!)
Make sure you keep the rest of your makeup minimal with waterproof liners and mascara, cheek and lip stains and always, always, always have spf!
I like to throw in a UV protectant spray for my hair and scalp as well as lip balm and makeup setting spray!
Re: Beach make up
Spyski hit it on the head about something more natural. Getting something too full coverage will feel too heavy on your skin. If you have a full coverage foundation and don't want to buy a new one, mix a touch of oil free moisturizer in to thin it down or get a tinted moisturizer which I covered below. I know you asked about foundation, but being in Miami, you've got to take advantage of the beach and water!
Some of my beach basics/must-haves:
-Sunscreen, no need to go crazy expensive or high end, I normally grab a two pack of Neutrogena's SPF 45 or 55 for the body that costs about $13, I mix it with my body lotion in the morning and apply it all over and toss a tube in my tote if I'm going to hit the beach/pool. I reapply it on the beach every 90 minutes to 120 minutes if I'm in and out of the water constantly. Peter Thomas Roth makes a powdered sunscreen you can dust over make up to reapply your SPF without ruining your make up, or you can try a sunscreen spray.
-Waterproof mascara, I love Urban Decay's Cannonball, you can swim, sweat, add eye drops, take a shower, rub your eyes and it does not budge!
-Cheek/lip stain, simple to use, great to have in your bag to build up more color throughout the day or give a hint of color to your complexion after getting out from the pool, and long wearing. Tarte and Stila make some good ones!
-Waterproof, creamy eye liner, smudge it with your finger to have it multi-task as a shadow, Urban Decay's 24/7 liners or MUFE's Aqua Eyes are superb for this and come in an array of colors.
-Tinted moisturizer, much lighter and easier to apply after swimming/being on the beach, plus more forgiving if you pick up a tan so your make up isn't so full colored your face/neck look like two different shades.
-Lip balm with SPF, Jack Black or Anthony Logistics make great, thick balms that last long and hydrate while giving sun protection. My fave is AL's Blood Orange scent!
-Various bobby pins, twist pins, and bungee cord pony tail holders, I like the wider/more open bobby pins as they make pinning a bun much easier when you want to whip your hair into a quick bun, especially when being out of the water for a chic look. Goody makes these awesome corkscrew/twist pins that are great for simple french twists or buns and lock real well into hair if you don't want to carry a dozen loose bobby pins. The bungee cord pony tail holders don't pull or tug on hair as much when trying to do a pony, especially with wet hair, you wrap it around the base of the pony to the degree you want and it hooks together in place for a secure hold.
Re: Beach make up
Go for something natural, not full coverage.
I live very close to New Orleans so I understand what the humidity is like. Be prepared to use blotting papers if you have oily skin especially around the t-zone.
I would highly recommend getting a foundation with SPF in it. I'm currently wearing Tarte's BB cream as my foundation which is tinted and has SPF 30 as well as a mosturizer in it. It lasts my entire work day and I only have to blot my nose once after about 8 hours of wearing it. Set the foundation with a setting powder such as bareMinerals Illuminating Mineral Veil or Hourglass Ambient Lighting Powder . That will set the foundation in place and give a sheer luminance for a natural look.