I decided to offer up more precision in a few previous calculations and some entirely new data that should shed more light on the sea salt versus sodium chloride comparison. I first want to mention something about how salt taste receptors work according to the latest IUPAC publication on taste recognition chemistry. This information incidentally came to fruition after my college years during which I studied biophysics, so I'm going to present two models, but I would tend to agree with the latest research.
It was believed that phospholipid membranes carried a full-time negative charge through what are referred to as anion receptor sites. Without going into too much detail, the old model allowed complex ion interactions that involved ion pairs and triple ions, with the ion pairs being either fixed or mobile, but regardless of pairing, the sites always remained negatively charged. It was a purely electrostatic model, and the primary source of saltiness came from anion activity (e.g. sodium, calcium, magnesium). From this we could conclude that by adding salts such as calcium chloride, magnesium sulphate, magnesium chloride, and strontium chloride, sea salt would gain a considerable amount of saltiness over just sodium chloride since calcium, magnesium, and strontium all have 4 times the ionic strength of sodium. Using the Department of Energy's sea water molar ion concentration data, that translates to 1.545856472 times saltier.
Today, the understanding of how taste receptors for salt (and sour actually) work is a bit different. The funny thing about the new model is when I was learning about the old model, I thought that it was strange to consider the ionic strength a gauge for saltiness when I knew for a fact most "salts" don't taste like sodium chloride. Flashbacks of those thoughts from years ago came to me when I read Evelyne's 2x salty statement. The reality is that sodium chloride is the only salt we unconsciously reference when we think of something being salty, so adding anything else to sodium chloride should in theory diminish the saltiness. Despite this, I will still present the "saltiest case scenario" based on ionic strength, but point out where deviations must be accepted based on the new model.
The new model adds bilateral symmetry to the equation; so along with ionic strength, the size of the ion and electromotive potential are taken into consideration. According to this model, sodium chloride is actually the saltiest salt in existence because it has the highest degree of bilateral symmetry. All other salts under this model are predicted to give a metallic or bitter flavor. In fact, many people describe the common substitute, potassium chloride, as having a metallic taste. From this we could conclude that sea salt is not as salty as sodium chloride, but instead simply has a stronger flavor than sodium chloride. Since this model is new, and so many people insist that sea salt is a lot saltier than regular salt, I've allowed for some liberal calculations that could compensate for possible errors in the new model, but at the same time remain compliant to the general framework. Here are all the possibilities from both the old and new model:
1.1015831251, based on anion and cation strength (+/-) and bilateral symmetry
1.1334652758, based on +/-, selective solubility, and bilateral symmetry
1.1586744343, based on +/-
1.1922089285, based on +/- and selective solubility
1.3067558171, based on selective +/- and selective solubility
1.5458564720, based on +
1.5905968351, based on + and selective solubility
Keep in mind that these numbers are based on calculations made from the U.S. Department of Energy's Oak Ridge National Laboratory sea water molar ion concentration data. Regionalized sea salts from around the world may vary, but this is the most complete dataset I had to work with. If anybody out there has a specific sea salt that they want me to analyze and provide a "saltiness factor" for, just let me know the individual salts contained in the sea salt and their percentages. I know that information is hard to find. If it were easy to find, I would have offered an example of a regional salt as a comparison to the DOE salt already. It's up to you if you really want to know how salty the salt you use is.
The other update I wanted to make is to the percentage change in sodium when switching from sodium chloride to sea salt. The 25% I stated earlier was an approximation because of data uncertainty. Based on this latest DOE data, the exact change in sodium levels is -22.06264313%