Look up "water jet cutting". You can cut steel, glass, rock, all kinds of stuff... with water! This is much more effective with abrasives added to the water (sand, garnets, etc).
I read a superhero book a few years ago where one of the characters had almost complete control over his body. One of his techniques was basically a blood chainsaw/whip thing with his own blood (rapidly joined by the blood of anyone hit hit with it, but he could easily filter out anything he didn't want in him when he was done).
If they can control the temperature of their own bodily fluids, they basically can't get too hot or too cold. Sunburn, but never heat stroke, never frost bite. They need never sweat (while awake and actively controlling their temp). A sandy water umbrella would be handy. Rip the fluid out of a cactus, mix with sand to reduce transparency, tada!
Depending on just how much effort boiling water costs them, this person might be able to power a steam turbine on their own for a time. Nuclear reactors are just steam turbines with a heat source that isn't a fire.
Just how cold can they make stuff? Dropping the sweat on someone's skin/tears in their eye to absolute zero would cause Serious Harm or Death.
Ice gets harder as it gets colder.
See also "Pykrete". Water (86%) mixed with sawdust (14%) and frozen has twice the crush strength of ice, 4 times the tensile strength of ice (and twice that of concrete), and the sawdust is also an excellent insulator, slowing down melting. The US looked into building a rediculously big aircraft carrier in WWII using pykrete.
Apparently using newsprint paper instead of wood pulp makes pycrete twice as strong. (Mythbusters episode 115).
Just how hot? "I cast sweat to plasma!"
Alternatively, press a relatively small amount of water (a teaspoon maybe?) against someone, convert it all to steam... But only let the steam escape through a tiny hole in that volume pointed into the person. "Water jet" from above, only with steam.
Alternative to the alternative... whip a chip of ice into someone's chest, and then convert it to steam only now its inside them. Water expands in volume by something like x1600 when converted to steam at 1 atmosphere of pressure.