Yes, cast iron utensils can be used to boil water. However, don't boil water for over 10 to 15 minutes to preserve the seasoning layer. Cast iron utensils can endure temperatures of up to 1500°F and water boils at 212°. Thus, boiling a small amount of water won't harm the cast iron utensil. But, without seasoning, the cast iron utensil can rust. In comparison to newer pans, older pans will have a thicker seasoning. So use caution while boiling water in a newer cast iron utensil.