man is there some bad information in this thread!
STRESS COAT, STRESS COAT, STRESS COAT AND MORE STRESS COAT!!!!!!!!!
I cannot even begin to tell you how bad of an idea this is . . . as soon as you turn on the skimmer, you will have nothing but bubbles and foam in your tank for days possible even weeks. I would never use stress coat or any similar product in a marine tank.
Most salt mixes will make tap water safe so you should be fine.
Wanna expand on this a little more? I don't think that salt mixes will neutralize/eliminate chlorine and chloramines . . . could be wrong though. It will stabilize pH though.
dump the salt directly into the tank, then add water. put a couple power head in and your on your way.
This isnt all that bad of an idea however doing so can cause beneficial elements to precipitate out such as Ca ions. Its better to add the water first and then dump in the salt. An even better way is to put the water in and slowly add the salt. I know that Ca and Alk are not of high importance in a cycling marine tank however, why create unstable conditions from the start.
To answer the original question, you could use tap water to fill the tank for the first time, however I would strongly recommend checking with the utility company first and then make your decision based on what they tell you. I did start my tank with tap and then use RO for WC's and top offs, however should I start another tank I will probably not take this route again. Using tap water can lead to massive algae blooms that can last a very long time. Granted that using RO will not eliminate this possibility, it will greatly reduce the severity.
Here is my solution . . . buy an RO filter. With a 210 gallon or whatever tank, the amount of money that will be spent on buying RO water for top offs and water changes will outweigh the purchase price of an RO filter in no time. Do it right from the start . . . buy the filter now and use RO to fill the tank, it may take a couple days to fill, but if you dont have the patience then you are in the wrong hobby.