Ziploc bags are not recommended to boil food in because the Ziploc bags are not made to withstand the hot temperature of boiling water.
Boiling water is 212 F degrees and above and Ziploc bags are only rated to stand a temperature of up to 195 F which is when the Ziploc bag begins to soften.
Ziploc bags are made from polyethylene plastic with a softening point of approximately 195 degrees Fahrenheit.
So if you place regular Ziploc bags in boiling water the Ziploc bags could melt and ruin your food that you're boiling it in.
You can however by plastic bags that are made for boiling food it and they are made to withstand the boiling water temperature.