Focal length of lens in a medium is given by

$\frac{1}{f}=\left({\mu }_{\mathrm{rel}}-1\right)\left(\frac{1}{R_1}-\frac{1}{R_2}\right)$

The relative refractive index of a medium is more in air and less in water

${\left({\mu }_{\mathrm{rel}}\right)}_{\mathrm{a}\mathrm{i}\mathrm{r}}>{\left({\mu }_{\mathrm{rel}}\right)}_{\mathrm{w}\mathrm{a}\mathrm{t}\mathrm{e}\mathrm{r}}$

$\therefore f_{\mathrm{a}\mathrm{i}\mathrm{r} }< f_{\mathrm{w}\mathrm{a}\mathrm{t}\mathrm{e}\mathrm{r}}$

Focal length of a mirror does not depend on medium.