hi Izrul,
just to comment: Have you consider that in surface, coastal area and offshore area, the salinity will change depend on the affecting environment; also not to mention the seasonal factor. salinity has no any correlation for depth estimation. In my opinion, the salinity can not be used as one of the parameter in your algorithm to produce water depth information from SPOT 5.
But, as always, I'm open for discussion.
bsrgds,
dnandz