Far Cry 3 runs fine with my i5 2400 and Radeon 6870 except when I look out to the ocean. The frames immediately drop from around 30-50 to single digits. I have the latest drivers installed, 12.11 CAP 2 and the latest game patch. I tried lowering the settings but there is always that extremely annoying fps drop.
I think the water might be using DX11 Tessellation, which is usually pretty ok but this game isn't optimised too well so Tessellation can have a great effect on performance. Not much you can do except turn down the water quality.