jradan,
Large bass in clear water can be very tough. I'd suggest that, if you can see them, they likely can see you. Large bass are old and experienced fish. An 18" smallie here in central PA is likely well over a decade old and has seen everything. My advice would be to back off when you see 'em. If a large bass is holding on a certain piece of structure or cover today, he'll probably be there tomorrow (gradually they'll move deeper as summer arrives). If you see a big fish, make a note of his location, return a day or two later if possible, and then sneak up on the same spot without getting too close. Make as long a cast as you can, ideally under low light conditions. The make/model of fly you use probably won't matter - just don't let that fish see yuh before he sees your fly.