You're confusing melting/boiling with thermal decomposition. One way you can control the heating is with a double boiler. I.E. water boils at 100C, so if you used a double boiler you won't exceed 100C.
Problems may occur from uneven heating. When the kettle bottom becomes much hotter than the water. Area of the kettle may get extra hot and flash boil the water into vapor. These can be observed and bubble rising to the surface.
You can avoid uneven heating by using boiling stones, or stirring.
Edit:
DMT, has a very high boiling point. 340-350C. DMT acetate has a significantly higher boiling point. The temperature of the water (100C) would act as a condenser in the event that much dmt was being flash boiled. Especially in the event that water was being improperly heated, and that the majority of your vapors exited as gas bubbles from the bottom.
To boil off water more effectively you need to stir the solution so that it heats evenly, so vapor could rise off the entire surface area of the water rather than intermitent bubbles.