An 8wt will be okay but a 9 or 10 would be the best fit. If you're chasing the smaller guys (10lbs or so) you'll be okay. Floating line is probably best, but if you have to use an intermediate, don't use a weighted fly. I would either build a leader of 60-40-20 or just use a level leader of 20-40lb fluoro. Wear them out before you get them on the boat because they're serious thrashers when landed.
The flies I've had the best cobia success on are any combinations of chartreuse, red, white, and pink on a 4/0-6/0 O'shaughnessy hook. Anything tied for pike or muskie will work. They like big, bright, flashy flies that are on or close to the surface (think flat wings, big sand eel patterns, large mushmouths, kinky muddlers, tabory snakes, etc.) because you'll be sight casting to them sitting up really high in the column, if not on the surface. They're not very picky on fly patterns - if they're eating, they'll eat anything.