Summit Art Glass Co. certainly do make Carnival glass (using both old and new moulds). However, their trademark is a V (usually) in a circle. They don't use it very often. Glass made by Summit can also be found carrying the Westmoreland mark, and sometimes the Imperial mark - as sometimes the previous company's mark has not been removed. Mostly Summit's Carnival is unmarked.
The only company I know that makes / made Carnival in water set shapes, and has an S trademark is Smith. But as I say, it's not in a diamond.
The only trademark on Carnival that uses the diamond shape is Boyd's B (in a diamond). But they don't make water sets.
Guernsey (B mark) also made some Carnival water sets.
These are the only trademarks I can think of that it could be. I know how marks can get distorted in manufacture and through use. In fact I have even seen a couple of Boyd's novelty items (a scotty dog and a crinoline lady) on eBay recently - the seller described the mark as an old English diamond registration mark.