7 dissolved salts in sea water

As each anion is paired with a cation, then the number of dissolved salts is 2ⁿ, where n is a zero or positive integer.

As 2ⁿ = 7 has no integer solution for n, the posted question is abstruse.

To be pedantic, there are no salts in the sea. Once you dissolve salts, you have a solution or electrolyte. So, there is Na⁺ and K⁺ and Cl^- and so on.

As to earlier misdiscussions, you cannot dissolve cats in the sea, only in carbonated water (which gives you catatonic).

Don't be daft of course they are salts and the concentration of each ion will need to be below the saturation point otherwise they will precipitate.

Where does the cryptic "7" come from? That sounds like folklore. They will be chlorides, sulphates, carbonates, iodates, bromates, silicates, aluminates ferrates amid probably others. They will be salts of sodium, potassium, calcium, aluminium, iron and other metals.

Maybe the OP is thinking abut the "Seven Seas"?

http://en.wikipedia.org/wiki/Seven_seas

There are: seven deadly sins, seven cardinal virtues, seven wonders of the world, the castle of seven towers, the house of seven gables, lucky number seven, seven-eleven stores, sevendust, and probably many more.......

Seventh heaven, seven pillars of wisdom.....