I believe the second sentence suggests untruth in that it might be interpreted that once water is boiled it will continue turning into steam irrespective of whether it is still hot.
This example should illustrate what I mean: "When wood is barked, it dries faster"
To make reference to rapid evaporation of water under certain conditions I would write:
"When water is boiling, it turns into steam"
"When water is being boiled, it turns into steam"