Rinsing canned vegetables with water can remove some of the sodium, but it's difficult to know exactly how much. I'd say in the range of 10 to 30 percent of the sodium can be reduced if you rinse your canned vegetables for at least one minute.
It's not difficult to do. Pour your vegetables into a colander and place it under running water for one minute. Allow the vegetables to drain, and they're ready to use.
Keep in mind that studies and sources seem to differ a bit in how much sodium can be washed away, so if you need to track your sodium grams every day, you're probably better off buying low-sodium canned goods. Read the Nutrition Facts labels on the backs of the cans, it will tell you how much sodium is in each serving (also look for the number of servings per package).
Nutrient Data Laboratory, Beltsville Human Nutrition Research Center, Beltsville, MD. "Effect of Draining and Rinsing on the Sodium and Water Soluble Vitamin Content of Canned Vegetables." Accessed August 11, 2013. http://www.ars.usda.gov/SP2UserFiles/Place/12354500/Articles/EB11_DrainedVeg.pdf.
Vermeulen RT, Sedor FA, Kimm SY. "Effect of water rinsing on sodium content of selected foods." J Am Diet Assoc. 1983 Apr;82(4):394-6.