I know that water benders can freeze and melt water/ice, but what is the limit to this?

Can a water bender heat water to boiling? Steam? I know that water benders can bend steam (Korra did when she went to the anti-bending rally), but can she create the steam if needed (without combining her fire bending)?

If not, why? If they can freeze water and then cause it to heat up again, what is stopping them from heating the water further?