Cherenkov radiation[0] is when a photon is travelling faster than the speed of light through water. To overly simplify, the speed of light in a vacuum is different than the speed of light through a medium.
> For example, the refractive index of water is 1.333, meaning that light travels 1.333 times faster in a vacuum than it does in water.
Light travels at about 0.75c in water.
You are both correct, but on different levels.
No, in water, the spacetime is not significantly warped, certainly not warped enough to slow down light. The different speed of light in a medium compared to vacuum comes from the interaction between the photons and the medium. The photon then, effectively, becomes a hybrid "photon-medium excitation" which is an effective particle with an effective speed lower than the vacuum speed of light.