Is human vision sensitive to frequency or wavelength?
It is not the wavelength or frequency that determines light absorption- it is the energy of the photon that matters. The energy of incident light should match the excitation energy of the chromophore. The medium itself can also affect light absorption by electronically interacting with the chromophore. Frequency is proportional to the energy (given by the relationship $E=h\nu$); while wavelength and velocity change in different media, frequency and energy remain constant. 

The chromophore in the opsins (retinal) absorb energy to become active and you can say that they detect the frequency. However, sufficient number of opsins have to become active for the retinal cells to transmit the signal. So the overall light flux is also important. An interesting thing to note is that activation of the opsins hyperpolarizes the photoreceptor cell and reduces its tendency to fire.

Wavelength plays a role in diffraction and therefore diffraction is less in a medium with higher refractive index. Wavelength also determines scattering. So, while the opsins actually detect frequency, the wavelength can slightly affect the overall "vision" because of scattering/diffraction effects.