Since I don't know the exact intensity of the heat present at the wedding (which would depend on where you stay), the textures that would suit best for your dress are crepe, charmeuse, chiffon and organza. You can choose from any of these while buying your summer dress. Chiffon by far, is the most commonly used material for beach wedding dresses for guests. Summer dresses are meant to be light in contrast to winter dresses that are usually more full and thick in their texture.
Floral prints are the best for summer dresses while plain print combinations stand second. The density of the prints again, have to be very light rather than intricate styling. Summer dresses are meant to look plain and simple, yet very elegant. You can also opt of a summer dress that has a variation of shades than just one color. The print has to look soothing to the eyes rather than something that would stand out and be an eyesore. You can also ask a stylist to show you some prints, if you're getting your dress stitched.
One of the most important aspect of a summer dress is its pattern. Every type of dress has a pattern that needs to be followed. Try to wear something that is knee length rather than a full length dress. A sleeveless or balloon sleeve dress is preferred. Also, a cut here and there would make sure the heat doesn't trouble you much. Wear something that is not very tight for its fitting as this would mean discomfort most of the time with the continuous sweat.
You must have heard of color schemes for summer weddings and winter weddings. Similarly, there are color schemes you can choose from for summer dresses and ones that you should avoid. Some of the most preferred colors for summer dresses are pastel, sherbet lemon, mauve, lilac, white, lemon yellow, light baby pink and shades of golden. You can also wear a combination of these. Avoid all dark colors as they won't only be too wrongly bright but will also absorb more heat.
Accessories on summer dresses should be delicate and elegant. Don't wear necklaces that are very heavy and thick in their presence. Small beads or pearls are a great option for necklaces. If you wear a watch, wear one with a delicate belt than a thick one, the color again one of the mentioned above or plain silver/gold. Earrings again have to be delicate and not very huge. Avoid wearing too many even if you have piercing. Just one set of earrings should be fine. The colors of all accessories should be in accordance with the color of your dress.