There is no magic pill that will make a truly overweight person look skinny at a beach. A flabby body is going to be a flabby body no matter what you do until you take the weight off. The next best thing is to try some appearance tricks that don't emphasize your extra pounds.
Invest in a slimming swimsuit. Some companies specialize in suits that minimize body parts. If you're heavy in the hips, look for a suit that targets the hips. If the weight is the stomach, get a suit designed to flatten the stomach area. A suit with an attached skirt could help you cover heavy thighs.
Pick a color that won't draw attention to your weight. Choose dark colors over bright colors. A woman carrying weight in the lower-half of her body should stick to solid colors from the waist down.
Create a tanned look, which can at least give your skin a healthy appearance. Try a tanning lotion or at-home spray tan can before you go to the professionals.
Stay away from white clingy fabrics. They will only accentuate your imperfections.