The ocean is about 3% salt. This varies a bit though, depending on where you measure it. Near where rivers reach the ocean, the salt content (salinity) will be lower. In fact, because salt water is more dense than fresh water, the river's water will actually "float" on top of the ocean water until it is mixed together. So if you took a reading near the surface of the ocean where a river flowed in, the salinity would be lower than that near the bottom.

Evaporation takes away water, but leaves behind salt, so in places where there is a lot of evaporation, the surface water may be saltier. Where do you think the saltiest water might be? You might want to look in the ScienceLine archives for my answer to a question about salinity in tidepools.

What a great question!
Here's how you'd find your answer:
Filter some seawater to remove all the tiny animals and bacteria swimming around in it, fill a plastic container with seawater, weigh the container and seawater, wait for the water to evaporate and then re-weigh the container and the salt left behind.

You can then calculate the percentage of seawater that is salt by weight. You'd need a precise scientific scale(your bathroom scale won't work) because only a small fraction of the total weight of seawater is salt -- about 3.47% on average.

Oceanographers call this percentage "salinity", as in: "Seawater has a salinity of 3.47% on average." Because it's so small, oceanographers actually measure salinity in parts per thousand instead of parts per hundred ("per mil" instead of "per cent"), so the average salinity of the ocean is 34.7 parts per mil.

Some oceanographers dedicate their life to measuring very small changes in salinity, since this can affect large-scale ocean circulation patterns and can also give valuable information about changes in rainfall and storm patterns. In fact, small changes in salinity are what first alerted scientists that global warming has already caused large-scale changes in the Pacific and Atlantic Oceans.

Of these three bodies of water, how do you think they rank in terms of salinity, from most to least? What do you think makes these differences?
a) Atlantic Ocean
b)Mediterranean Ocean
c) Pacific Ocean
Answer:
b) The salinity of the Mediterranean is about 38 parts per mil.
a) The Atlantic Ocean is about 34.90 parts per mil on average.
c) The Pacific Ocean is about 34.62 parts per mil on average.

You mean the ocean's salinity? A quick Google search came up with the salinity of the ocean being about 35 parts per thousand, or about 3.5%.

In any case, I can suggest an easy way to find out. Get a cup or a bowl or something and weigh it. Then go down to the beach and fill it with water. Bring it back and weigh it again. The difference in weights is the weight of the sea water.

Now, put your bowl out in the sun and let it dry. When all of the water has dried up, weigh it again. The difference between the bowl's original weight and its weight now is the salt. You now have the weight of the salt and the weight of the water, so dividing the amount of salt by the amount of water will give you the weight ratio of salt to water in the ocean.

Sea water varies in its salt content from place to place. For example, there is a very low salt concentration in areas where a large river flows in to the ocean, due to high mixing with freshwater. However, in the open ocean away from land the salt content (or salinity) of the seawater tends to range between 30 and 35 ppt (parts per thousand). This is equivalent to 3.0 to 3.5 % (or parts per hundred). In hot areas where there is a lot of evaporation the number tends to be higher, whereas in areas that receive a lot of water input (such as from rain) the number will be lower.