Six Sigma is a quality control process where you try and get the number defects in a product to the "sixth standard deviation" (sigma representing the greek letter for standard deviation) or in other words, 0.00044% of parts are defect-free. From my industrial processes class ( 6 years ago now) it's best applied to large throughput items where you can accurately measure deviations over large sample sizes of the same item. (it was developed by Motorola to improve cell phone quality for example).
Lean Six Sigma looks to reduce waste in the manufacturing process. I think that's the difference. We aren't manufacturing a ship, we're constructing one. The very description implies more one-off methods and craftsmanship vs industrial processes. I'm sure some of the procedures can be applied to AOPS (the pipe shop or steel cutting for example), but a holistic look at the entire process shows other QC methods are better in many cases.