you dont want the dent, and you dont want to fill it fully with water. that is what i mean by carefully, you can to leave it a little empty, like fill it up and then put your finger into the hole to push out some water.
too much water, it will expand too much and rip a weld. not enough water and it wont expand enough to pop the dent out.
you also want to slowly push it out so it doesn't weaken too much.