You wont see any mayflies this time of year on Lake Semilnole. This time of year, I would throw diving crank baits, various soft plastics and maybe work in a spinner bait but your best bet will be crank baits or any billed baits. Find the back waters up there in the GA side. I would probley try to stayoff the River as much as possible.
You might want to try to go by Fairchilds(sp) on the west bank of the Chattahochee River away's up river from the Jim Woodruff Dam. There might be bait there and you can pull right up to it on your boat. If not, there is a place called Paramores just on the Florida side that has a marina and bait, food ect.
I was just up at Paramores last weekend and they have closed that restaurant/bait shop. There is a small tackle store right down the street from three rivers state park. I have fished the lake many times and caught some nice bass/crappie. The camping is awesome. The trees around the lake are huge and very scenic.