diffraction - Is the decrease in wavelength proportional to the reduction in speed?

I conducted a small at home experiment where I compared a red laser’s wavelength through water vs air. I put a diffraction grating in water and then did the same out of water. See photo It was difficult to measure the wavelengths accurately due to the short travel distance, but it is clear that the under water separation is smaller. I got 507nm for the under water wavelength, but as I mentioned above I question its accuracy. Should the wavelength be 487nm? This with the original frequency would produce a speed of 225,000,000m/s, light’s speed in water.