The water needs to be boiling. Perhaps this method is not appropriate for your use. (We need to match the method to the situation.) I assume that you are asking about this because you don't want to use RoundUp or the equivalent? How about an inexpensive propane weed torch, which would be perfect for this type of use?

Well, I guess I could just use my regular propane torch on a long stick (to avoid bending over for long periods).

Yes, you're right. I hate using chemicals, no matter how safe they purport to be. I have one large area (like 20x20 where I'd like to kill *everything* so I can start over with some alternative ground cover. I thought maybe the scalding water repeated once a week for a couple of weeks might do the trick.

I just *hate* using chemicals, especially herbicides.

Y'know, my brother has one of those "instant hot water" dispensers in his kitchen. I wonder how hard it would be to rig up something like that...

Take a large pan, put on stove, fill with water, turn on heat, let water come to a boil. Use a smaller pan to dip water from large pan. Either put in bucket or other container and carry to site. Turn stove off. Very Carefully take dipping pan and fill with some of the very hot water. Lower dipping pan to about 1/2 to 1 inch above plants. VERY Very carefully pour away from you the hot water on the plants. Repeat. The above instructions should be used with caution. No animals or children should be present when following this plan.

This is how it was done before hot water heaters and instant hot water. Do not turn your hot water heater to a higher temperature to heat water to kill plants. Turning the heat up on the hot water heater could cause the safety valve to release sending hot water every where.

Will this method work on ivy or blackberry?

Does it kill the whole plant... like DEAD? Or just what actually comes in contact with the water, leaving live roots to re-sprout?

Boiling water will generally not affect roots of perennial weeds. It is not an effective control for plants such as English ivy and blackberry.

Take an electric kettle to the garden (with outdoor extension cord), and a hose. Fill, boil, pour, refill, boil, pour, etc. I've killed thimbleberries this way.

Boiling water causes the cells in the leaves to burst so they die. Hot water needs to be over 200 degrees, almost boiling, to be very effective. While both are touted as organic methods of "weed" control both require the use of large amounts of non renewable energy, just as flamming does, so whether they fall into the concept of sustainability that organic gardening/lifestyle is very debateable.
The hot or boiling water method only kills off the top growth of plants not the roots, so some "weeds" can regrow and need further treatment. I have found digging unwanted plants is the most effective method of control as well as the least costly.