Do all decomposition reactions involve heat?
Plainly put: no, decomposition reaction does not always have to entail providing heat. A decomposition reaction (in simple language) is when one compound breaks into multiple other compounds, like so:

$$\ce{A -&gt; B + C}$$

As a general rule, if the product side is relatively more stable, it will decompose without you having the to provide extra heat. In the case of the reaction you gave, $\ce{H2O}$ is more stable than $\ce{H2}$ and $\ce{O2}$, and thus you will have to provide heat for the breaking of bonds.