I'd ask the people who are throwing it... but personally I'd wear a good pirate costume. Nothing too sexy or gaudy or cheesy (it is a wedding after all) but if they want you to dress up, dress up!

MEN:

Fancy Pirate (Captain Hook):
The rock-solid foundation of this outfit is a jacket from the late 1600's through the mid-1800's. The fancy pirate jacket we picture most is akin to the French revolution it has gold buttons and large cuffs. Wear it the the similar length vest underneath and a jabot necktie. The shirt collar is flipped-up under the tie. The wig is long and curly like Cher in the 80's, best if it's conspicuously parted in the middle. The optional hat is a 3-corner or a "taco" shaped hat. The third choice is any wide-brim hat with the one side pinned-up--or for originality, a Pilgrim hat. This can then be turned into an outlaw by featuring a braided beard and flamboyant jewelry. The best cliché being the hook for a hand or a wooden leg. Shoes with pilgrim buckles. Gold tooth.
homepage.mac.com/randalala...lbum1.html (first image)
www.bbc.co.uk/schools/gcs...tsrev3.shtml
www.ohjohnny.net/lib/roche...monkey.jpg

Other fancy options are any nobel dress from the period: Amadeus through Napoleon. Also military wear, a la Adam & the Ants.

Less dressed: The Filthy Crew
Any billowy shirt with long black or brown vest in leather or corduroy. Scraggly hair. Eye patch. Bristly face. Scars. Pants cropped at the knees tucked into striped hose. Motorcycle boots. Red or brown bandanna on the head.

The Scurvy Knave:
Short pants; either knickers buttoned at the knee, or wide gauchos. Shirless. Tattoos. Bald head or striped stocking cap. Blacked teeth. Dirt. Barefoot.

Gold hoop earrings all-around. The best boots are cuffed.


WOMEN:

Fancy Ball Gown Gone Bad:
Romantic period like Lilly Munster or Pinky & Blueboy (think: Empire Waist). Extra jewels, Hair in Sausage curles (Shirley Temple or Civil War curls). Dripping with pearls and rhinestone jewels. Belly-Dancer make-up. Or anything from Marie Antoinette through Civil War.
www.britannica.com/EBchecke...Elisabeth
flickr.com/photos/pantufla/1130420623/

Galleon Wench:
That ruffled top which can pass for Gypsy, Pirate, Renaissance, or Mexican Waitress. Waist cinch (or corset www.verymerryseamstress.com/pict...4.jpg ). Layers of long scalloped skirts--the top skirt is pulled up on the sides to make big hips. Powdered face and red cheeks like a French Revolution Whore. Ballet slippers, capezios or lace-up moccasin boots. Curley Cher wig. (but watch out for anything that sells it as a Gypsy or Mexican Waitress, like red-white-green stripes. or head scarf. www.folkloremexicanodsdc.org/pho...html )


ILLEGAL:
Sexy Pirate Wench™ costume from a package. Commercially produced "Jack Sparrow" costume.
TOO CLICHE:
Sculls, parrots, red velvet goth outfits.

Hope that helps. Show me the picures!

Try Sequoia.