Is there a kind of glue that would melt at water boiling point but
would be water-resistant at room temp? I need to glue together some
parts (glass and plastic) which I could later recycle by placing the
piece in boiling water and recovering the parts.
While not technically a glue, you could use Friendly Plastic:
to hold the parts together. This stuff is similar to hot glue, but
stiffer/harder at room temperature than hot glue. It softens at around
150F. One issue (which may be an advantage) is that it really sticks to
almost any surface. You may have to scrape off small bits off the
recovered parts, which will be an issue if you have very small,
intricate or fragile details.
Another option might be a wax, like Flexwax (which is a reusable mold
material that melts at 120F):
Or even just regular beeswax or a dental wax.
Hardens at room temperature.
Is water resistant.
Can be disolved with acetone.
Of cousre, you may have issues with the acetone reacting with whatever
crazy objects you're planning on glueing together.