I cannot guarantee the materials in this answer will meet all your requirements, so it would still be very helpful if anyone else could pitch in (especially an inorganic chemist or someone with hands-on knowledge of refractory materials). I will describe my thought process, which may ultimately be more useful than the suggestions themselves.
Immediately, the requirement to resist temperatures up to 1000 °C implies organic materials are very unlikely to be suitable. Almost all reasonable organic compounds would boil, decompose or burn (in air) well before then.
So now we focus on inorganic materials. Simple examples of very heat-resistant compounds are quartz ($\ce{SiO2}$) and alumina ($\ce{Al2O3}$), both melting only past 1500 °C and with no decomposition. However, these materials also resist dissolution; quartz dissolves only in concentrated hot aqueous base, whereas alumina dissolves in both of concentrated hot aqueous base or aqueous acid.
That said, it is possible to add other compounds to these materials and change their properties. For example, mixing $\ce{SiO2}$ with a number of other inorganic substances causes its melting point to lower to more manageable temperatures. This is how we make most kinds of glass. Particularly interesting is that the addition of sufficient alkali metal ions can turn the glass water-soluble. For example, there are a variety of sodium silicates, whose exact composition and properties will vary with the sodium content, and at high sodium content they become very water-soluble. However, it appears these silicates melt close to 1000 °C, which may preclude their use.
Fortunately the same approach works with alumina; increasing its alkali metal ion content eventually renders the material highly water-soluble. Sodium aluminate with a 1:1 mol ratio of sodium to aluminium forms $\ce{NaAlO2}$, which appears to be very water soluble and still melts at >1600 °C. In principle, this material could be made into a concentrated, thick aqueous solution or slurry, poured into a mould, dried at low heat to solidify then calcined to obtain a sodium aluminate object of the desired shape. This could then be used for your applications, and redissolved in water when necessary.
In both of the previous examples, it's certainly possible to add further inorganic compounds (e.g. borax, lime, etc.) to tweak properties further. However, much of this kind of research has already been performed, so I would recommend digging into glass fabrication/cement research rather than rediscovering the wheel.
I should also mention there are ways in which my above suggestions may be inadequate. Alkali aluminates/silicates appear to be strongly hygroscopic, which may make their use more cumbersome. More worryingly, at high alkali metal content they become strongly basic, which could corrode some of the metals you plan to use, both during dissolution in water, or by direct reaction with the aluminate/silicate surface at high temperatures. You'll have to see whether this is a potential deal-breaker.
