In high enough amounts, salt water is toxic and will kill most plants that do not naturally thrive in it, such as seaweed, but some salt in water is natural and some studies show it may even be beneficial.
All water has some salt in it, but too much salt in water can be dangerous for plants. According to biologist Keith McGuinness, this is because salt water actually sucks water out of the plant roots, as opposed to the roots absorbing the salt water.
Scientists at the University of Pisa in Italy found that tomatoes watered with a solution of 12 percent seawater produce more antioxidants like vitamin C and dihydrolipoic acid than tomatoes that were watered with plain fresh water. The tomatoes did not grow any faster or larger, however.
If you suspect the water you are using to water plants has a high salt content, examine the edges of plant pots that you are watering. Evidence of a high salt content can be found in the form a white or tan crust along the edges of the pot. If this is the case, you may want to run the water through a water purifier before watering your plants.