Why iron(II) spectrometry is not measured in distilled water and it requires a complexing agent like o-Phen?
I'm sure OP get the answer by reading all comments and the answer elsewhere. However, I'd like to point out few things on this spectrophotometric determination of $\ce{Fe^2+}$ in drinking water (it is not distilled water):

Iron can exist in natural waters where it originates from the dissolution of minerals. Even though most iron containing minerals are not very soluble in water as evident by the solubility products of iron-containing salts. However, a trace amount of iron can still dissolve in water. The dissolve iron is predominantly found in the form of $\ce{Fe^3+}$, because dissolved oxygen in water oxidizes $\ce{Fe^2+ -&gt; Fe^3+}$.
Based on above fact, water cant be green, but probably yellow. But, it can be still determined by UV-vis spectroscopy, only if you have a lot of $\ce{Fe^3+}$ ions in your unknown drinking water sample to do so (see Ivan Neretin's comment and Maurice's answer), which is not possible.
On the other hand, 3 molecules of 1,10-phenanthroline (ortho-phenanthroline, a bidentate) chelate with one ion of $\ce{Fe^2+}$ to give highly intense orange-red $\ce{[Fe(o-phen)3]^2+}$ complex ion in water (Ref.1). The equilibrium constant for this reaction $\left(\ce{Fe^2+ + 3 o-phen -&gt; [Fe(o-phen)3]^2+}\right)$ is $2.5 \times 10^6$ at $\pu{25 ^\circ C}$  in the $\mathrm{pH}$ range between  $\mathrm{pH} = 3$ and $\mathrm{pH} = 9$. Thus, a $\mathrm{pH}$ of about $3.5$ (using acetate buffer) is recommended to prevent precipitation of iron salts.
The molar absorption coefficient (ε) of this ferrous complex, $\ce{[Fe(o-phen)3]^2+}$ is $\pu{11,100 dm3 mol-1 cm-1}$ at the wavelength of maximum absorbance, $\lambda_{max} = \pu{508 nm}$. Thus, spectrophotometric absorbance of this complex can be easily measured in $\pu{10^{-4} M}$ to $\pu{10^{-5} M}$ or even lower levels at $\pu{508 nm}$, which is compatible with $[\ce{Fe^3+}]$ levels in drinking water (usually well water).
However, there is a problem: 1,10-Phenanthroline complexes quantitatively only with $\ce{Fe^2+}$ ions. But that can be solve using a reducing agent such as hydroxyl amine ($\ce{HCl}$ salt, which is soluble in water):
$$\ce{4Fe^3+ + 2HO-NH2.HCl -&gt; 4Fe^2+ + N2O + 6H+ + 2Cl- + H2O}.$$ The reducing agent should be used in excess to prevent $\ce{Fe^2+}$ oxidizes back to $\ce{Fe^3+}$ by dissolve oxygen.

References:

R. Belcher, &quot;The application of chelate compounds in analytical chemistry,&quot; Pure Appl. Chem. 1973, 34(1), 13-28 (DOI: http://dx.doi.org/10.1351/pac197334010013).