Let's analyze each statement to determine which is incorrect:

Option A: The electromeric effect is a temporary effect.

This statement is correct. The electromeric effect is indeed a temporary effect that occurs in the presence of a reagent and disappears when the reagent is removed.

Option B: Hydrogen ion $$(\mathrm{H}^{+})$$ shows negative electromeric effect.

To understand this, let's define the electromeric effect. The electromeric effect involves the transfer of electron pairs within a molecule in the presence of an attacking reagent. There are two types of electromeric effects: the positive electromeric effect ($$E^+$$) and the negative electromeric effect ($$E^-$$). A positive electromeric effect occurs when electrons are transferred towards the reagent, while a negative electromeric effect occurs when electrons are transferred away from the reagent.

Since a hydrogen ion $$(\mathrm{H}^{+})$$ is an electron-deficient species (it lacks electrons), it would generally induce the transfer of electrons towards itself, resulting in a positive electromeric effect. Hence, this statement is incorrect.

Option C: The organic compound shows electromeric effect in the presence of the reagent only.

This statement is correct. The electromeric effect is observed only in the presence of an attacking reagent. Once the reagent is removed, the effect ceases to exist.

Option D: Electromeric effect dominates over inductive effect.

This statement is correct. The electromeric effect generally has a stronger influence on the electronic distribution within a molecule compared to the inductive effect, especially when both effects are present simultaneously.

Therefore, the incorrect statement is:

Option B: Hydrogen ion $$(\mathrm{H}^{+})$$ shows negative electromeric effect.