Think of how easily light bends when transitioning between two very different mediums like air and water. If the surrounding medium becomes denser (like water instead of air), the contrast in optical density decreases, so light doesn't bend as sharply, stretching out the focal point.

According to the Lens Maker's Formula, the focal length depends on the relative refractive index of the lens with respect to the medium ($\mu_{lm} = \frac{\mu_l}{\mu_m}$). When the lens is placed in water, the surrounding medium's refractive index increases from $1.0$ (air) to approximately $1.33$ (water). This reduces the value of $\mu_{lm}$, thereby decreasing the refracting power of the lens and causing light rays to bend less, resulting in a larger focal length.

Always remember that the power and focal length of a lens are medium-dependent, unlike the frequency of light during refraction which remains constant.