Let's take an example of adding 10% of the mass of the Sun in the form of water. The water does not remain in the form of molecules. It is easily disassociated into atoms, then mixed in the solar convection zone and then ionised. This requires energy.

$0.1M_{\odot}$ of water contains $6.6\times 10^{54}$ molecules (about $10^{31}$ moles). With a dissociation energy of energy of 492 kJ/mol, it requires $5 \times 10^{36}$ J to produce $1.2\times 10^{55}$ H atoms and $6.6\times 10^{54}$ O atoms. These are mixed into the convection zone of the Sun, the base of which achieves temperatures of $2\times 10^{6}$ K. To raise the atoms to this temperature requires a further $3kT/2$ per atom, so about $8 \times 10^{38}$ J. The H atoms are comparatively easily ionised at these temperature (13.6 eV per atom) but oxygen takes a whopping 433 eV to strip 6 electrons (appropriate for temperatures of $\sim 10^{6}$ K). So to ionise the material takes a total of $5\times 10^{38}$ J.

You might think that adding mass to the Sun would cause it to assume the main sequence configuration of a more massive star, but it is more complicated than that. If you add water, then that is mainly oxygen by mass. Partially ionised oxygen is an excellent source of opacity and would significantly change the overall "metallicity" (anything heavier than helium) of the Sun. It would go from being abut 1.3% metals by mass to about 9%.

As far as I know, nobody has done calculations of stellar structure for such an extreme metallicity star. In nature the most metal-rich star are about 5 times the metallicity of the Sun (Do et al. 2018).

The best I can do is point you to calculations for metallicities of 5% calculated by Pietrinferni et al. (2013) and then you can interpolate (or extrapolate at you own risk) appropriately. It is clear from their Fig.3, that a metal-rich $1M_{\odot}$ star is 30% less luminous and has a 10% lower surface temperature than a solar-metallicity star of the same mass. It must therefore be slightly smaller too. However, we need to do the comparison with a $1.1M_{\odot}$ metal-rich star on the main sequence. You can directly compare a metal-rich model at $1.1M_{\odot}$ with a solar-metallicity model at $1M_{\odot}$. It turns out that the star with added metals is about the same temperature, but 20% more luminous than the Sun (and therefore must be about 10% larger). I caution you though that these are scaled solar metallicity models, they do not match the oxygen-rich nature of the added material here and they also include an increased He abundance to match the increased metallicity. I would also caution that I have not considered how well the metal-rich material can be mixed beneath the convection zone and into the core (the models assume the star is born from gas with that abundance).

Let's take an example of adding 10% of the mass of the Sun in the form of liquid water (in buckets, right, but let's also assume you don't throw the buckets in). The water does not remain in the form of molecules. It is easily disassociated into atoms, then mixed in the solar convection zone and then ionised. This requires energy.

$0.1M_{\odot}$ of water contains $6.6\times 10^{54}$ molecules (about $10^{31}$ moles). With a dissociation energy of energy of 492 kJ/mol, it requires $5 \times 10^{36}$ J to produce $1.2\times 10^{55}$ H atoms and $6.6\times 10^{54}$ O atoms. These are mixed into the convection zone of the Sun, the base of which achieves temperatures of about $2\times 10^{6}$ K. To raise the atoms to this temperature requires a further $3kT/2$ per atom, so about $8 \times 10^{38}$ J. The H atoms are comparatively easily ionised at these temperature (13.6 eV per atom) but oxygen takes a whopping 433 eV to strip 6 electrons (appropriate for temperatures of $\sim 10^{6}$ K). So to ionise the material takes a total of $5\times 10^{38}$ J.

You might think that adding mass to the Sun would cause it to assume the main sequence configuration of a more massive star, but it is more complicated than that. If you add water, then that is mainly oxygen by mass. Partially ionised oxygen is an excellent source of opacity and would significantly change the overall "metallicity" (anything heavier than helium) of the Sun. For an extra 10% of mass added in the form of water, the metallicity would increase from being abut 1.3% metals by mass to about 9%.

As far as I know, nobody has done calculations of stellar structure for such an extreme metallicity star. In nature the most metal-rich stars are about 5 times the metallicity of the Sun (Do et al. 2018).

The best I can do is point you to calculations for metallicities of 5% calculated by Pietrinferni et al. (2013) and then you can interpolate (or extrapolate at you own risk) appropriately. It is clear from their Fig.3, that a metal-rich $1M_{\odot}$ star is 30% less luminous and has a 10% lower surface temperature than a solar-metallicity star of the same mass. It must therefore be slightly smaller too. However, we need to do the comparison with a $1.1M_{\odot}$ metal-rich star on the main sequence. You can directly compare a metal-rich model at $1.1M_{\odot}$ with a solar-metallicity model at $1M_{\odot}$. It turns out that the star with added metals is about the same temperature, but 20% more luminous than the Sun (and therefore must be about 10% larger). I caution you though that these are scaled solar metallicity models, they do not exactly match the oxygen-rich nature of the added material here and they also include an increased He abundance to match the increased metallicity. I would also caution that I have not considered how well the metal-rich material can be mixed beneath the convection zone and into the core (the models assume the star is born from gas with that abundance).

Let's take an example of adding 10% of the mass of the Sun in the form of water. The water does not remain in the form of molecules. It is easily disassociated into atoms, then mixed in the solar convection zone and then ionised. This requires energy.

Short term effects

The best I can do is point you to calculations for metallicities of 5% calculated by Pietrinferni et al. (2013) and then you can interpolate (or extrapolate at you own risk) appropriately. It is clear from their Fig.3, that a metal-rich $1M_{\odot}$ star is 30% less luminous and has a 10% lower surface temperature than a solar-metallicity star of the same mass. It must therefore be slightly smaller too. However, we need to do the comparison with a $1.1M_{\odot}$ metal-rich star on the main sequence. You can directly compare a metal-rich model at $1.1M_{\odot}$ with a solar-metallicity model at $1M_{\odot}$. It turns out that the star with added metals is about the same temperature, but 20% more luminous than the Sun (and therefore must be about 10% larger). I caution you though that these are scaled solar metallicity models, they do not match the oxygen-rich nature of the added material here and they also include an increased He abundance to match the increased metallicity.

Conclusion

In the short term it depends on the rate at which you add the water and how you add it. The Sun could become more or less luminous as a result. In the long term the Sun will likely stay at the same temperature, will become more luminous, but not by the amount you might expect from a simple main-sequence scaling with mass, because the material you are adding is "metal-rich".

Short term effects

Let's take an example of adding 10% of the mass of the Sun in the form of water. The water does not remain in the form of molecules. It is easily disassociated into atoms, then mixed in the solar convection zone and then ionised. This requires energy.

The best I can do is point you to calculations for metallicities of 5% calculated by Pietrinferni et al. (2013) and then you can interpolate (or extrapolate at you own risk) appropriately. It is clear from their Fig.3, that a metal-rich $1M_{\odot}$ star is 30% less luminous and has a 10% lower surface temperature than a solar-metallicity star of the same mass. It must therefore be slightly smaller too. However, we need to do the comparison with a $1.1M_{\odot}$ metal-rich star on the main sequence. You can directly compare a metal-rich model at $1.1M_{\odot}$ with a solar-metallicity model at $1M_{\odot}$. It turns out that the star with added metals is about the same temperature, but 20% more luminous than the Sun (and therefore must be about 10% larger). I caution you though that these are scaled solar metallicity models, they do not match the oxygen-rich nature of the added material here and they also include an increased He abundance to match the increased metallicity. I would also caution that I have not considered how well the metal-rich material can be mixed beneath the convection zone and into the core (the models assume the star is born from gas with that abundance).