Lemons can be a fun catch. Blacktips and spinners are incredible. Another common shark on the flats is the nurse shark, which is near impossible to entice to a fly. The toughest part is casting a shark fly (often referred to as a half-chicken) with a bonefish rig. You'd be best off taking along a 10-12 weight and rigging it specifically for sharks. The key is the same as for most other flats fish. Cast it close enough so that the fish sees it, but not so close as the spook it. Sharks do not have particularly good vision so you really have to make the effort to get it close to them. Set the hook as if you are tarpon fishing and don't forget to bow if the fish jumps. Blacktips and spinners jump a lot. If you determine that the sharks are nurse sharks, don't even bother with them. Good luck with it and let us know how you do. I like to see sharks on the flats as that means there's likely bonefish, too.