Let me direct you to this Virginia Extension publication: Managing Wildlife Damage: Snakes......http://pubs.ext.vt.edu/420/420-021/420-021.html

It states..." A number of commercially available products have been registered by the U.S. Environmental Protection Agency (EPA) for use as snake repellents. All of the currently registered products contain some combination of pulverized sulfur and naphthalene as active ingredients, and most are offered in a shakable powder formulation. However, research conducted to investigate the effectiveness of these active ingredients in repelling snakes has yet to show any demonstrated success.

If you still wish to use the snake away product, and if it contains sulfur, it can reduce the pH of soil to a level that will be less than desirable (more acidic) for many plants including tomatoes. So you should apply it several feet away from the outer edges of the tomato plants to reduce the chance of altering the pH of the soil that the tomatoes are growing in. And remember anything you plant in the area where you use the snake away will be impacted by lower pH for future growing seasons.