"Alexa, start stop watch" or "Alexa, open stop watch". (do NOT say: "Alexa, start a stop watch." -> will not work)
When you activated this skill you can just say: "Alexa, Stop Watch."
The elapsed time is shown in hours, minutes and seconds.
Additionaly to the Alexa voice the Skill will show an historic picture of an old time-measurement device in the Alexa App, your Alexa Show or on https://alexa.amazon.com.
"Alexa, ask stop watch to pause." Will pause the stop watch. You can tell the stop watch to continue to run by saying: "Alexa, start stop watch."
"Alexa, ask stop watch for current measurement" Will tell you the elapsed time.