A crock pot is meant to slow cook food and doesn't really get hot enough to boil water so you won't be able to actually boil any water in the crock pot.
If you want to boil water you'll need to put the water in a pot and put the pot on the stove and turn up the heat until it starts to boil.
The water needs enough heat to get hot enough to boil and most if not all crock pots will not be able to bring the water to a boil.
I've never had any crock pots that would get hot enough to boil water so you'll have to boil the water on the stove in a pot.