Via E-Mail two years ago, Shimano advised me that the gold Calcuttas were designed for salt water use. I've had the understanding that the silver was for fresh water and the gold for the salt. At one time the silver cost about $10 less (if memory serves) than the gold version. Not sure they sell the silver ones anymore and I don't have any of the them, but if I had one I would not be reluctant to use it in the salt. Would replace the bearings with the anti-rust versions though. My understanding is that the silver is a cast frame and the gold is a forged frame and therefore stronger. At least this is what I was told when I bought my first ones about 10 years ago.
The gold anodizing was indeed intended for use in saltwater. Several customers still use the silver 200's in the salt with no problems as long as they have been properly maintained. The old 200's have been discontinued for a while now.