I'm wondering if it was possible for there to be a planet pretty much exactly like earth, except the ocean isn't salt water, it's somethingelse water. Can that happen? If so, how?
Water runs throught riverbeds and subterranean aquifers. In contact with soil and rocks, various salts and substances are dissolved in water. This water flows to the sea where it mix with the seawater. Substances dissolved in water are carried from land to the sea. To return to the river sources, water does not run upwards the riverbed as a solution. Instead, water evaporates from the ocean, rises to the upper layers of atmosphere and falls as rain. As the water has a much lower boiling point than most salts and substances - even if the evaporation of water is not a "boiling" per se - most of the evaporated substance is composed of water vapour, with very low ammounts of other substances (Iodine salts for example). So, you get a one way carry of substances from land to the sea, with a much smaller transport of substances from the sea to the land. This imbalance of transportation, increases the concentration of salts in water. The rate of evaporation and discharge regulates the salinity of water. Where a small ammount of water, feed by rivers (like say, dead sea) meets with high evaporation, large ammounts of salt concentrate on water. The ammount of substances that are carried by water are proportional to their solubility (in water - there are tables for this) and the availability of such substances on the riverbeds and aquifers that water passes throughout. This means that, besides the solubility, wich is a constant (under constant temperature and pressure) the other component that controls wich substances will concentrate at sea is the availability of them on the riverbeds.
Sodium chloride is a very soluble substance under the usual temperature found in our planet. Supposing a planet with same pressure and temperature as our own, table salt would concentrate in its sea. In order to change this, lets suppose that the planet in question has little to no disponibility of sodium chloride (table salt) in the sediments and rocks that form such planet. Even being very soluble, the low concentration of table salt in this planet will result in a sea with little to no table salt dissolved.
Solibility, availability and low evaporation - ie, the substance is unable to evaporate away with water in any significant quantity - are the factors that will determine the concentration of a substance in the ocean.
How to create a planet where the concentration of table salt is low and the concentration of other substances is high ?
Select a substance that you want to see in your oceans. Check its solubility (its usually expressed as a table where the rows are the substances and the columns are the temperatures). Find the best temperature - where the substance is most soluble. Set the climate of that planet accordingly to such temperature. Now change the rock and soil makeover to have high concentrations of that substance. Make sure the soil is low on other substances that might be carried away by the rivers. Verify if that substance, while dissolved in water, cannot evaporate away with water in any significant ammount. Verify if, under the planet climate needed to match the best solubility of such substance, there will be large evaporation of water from the oceans and subsequent rain over land. When all those criterias are meet, you get an ocean with a good concentration of the substance choosen.
You might verify that some substances are not soluble in water in any meaningfull ammount under a large interval of temperatures, or that the substance has low solubility and so on. While you can build your planet to match a certain substance as the one most present in its oceans water, you cannot change the solibility of the substance. So, in the event that the river brings more and more dissolved substance and the solubility limit is reached, the substance will start to crystalize or precipitate in the ocean floor. So, if your choosen substance has a solubility of 1 gram per 100 gram of water under 30 degress celsius, your ocean will have a max of 1 gram per 100 gram of water at 30 degress celsius, no matter what you do. In other words, you cannot foce a substance to match the ammount of salt in our ocean, because solubility is an intrinsic property of the substances (for each solute and solvent pair).
Real world example is a little more complex, because not all salt in ocean water came from rivers. Some came from sodium present in seabed being dissolved when the oceans formed, and some chloride from vulcanism (thermal vents). Its a complex process, but the general formula postulated in this post can be used to justify the creation of "somethingelse water" oceans in other planets that you need for your story plot.
You can have a sea of "substance X" water if the substance is soluble and the rocks and riverbeds of your planet have a good concentration of such substance and a low concentration of other, competing, substances.
What we commonly refer to as "salt" is actually a particular salt, sodium chloride. This is simply a matter of abundance, though. Everything (except the stuff that isn't found in nature at all) is found there, though: Concentration of elements in the ocean.