Very good site Svoberts!!
Sunil,
Take a look @ the cottonmouth pix. Does that look like what you have? Usually a common water snake will not hold it's mouth open, however that's precisely how the cottonmouth got it's name!
![eek](/forums/images/graemlins/default/eek.gif)
Both are agressive.
James,
After the snake is dead, you can positivly identify it as poisonous or not by looking @ it's eyes. All pit vipers in the US have vertical iris's. The only poisonous snake native to the US that doesn't is the Coral snake.