However, if the room is full of people then the actor can’t walk in and out of the room in a straight line. They keep moving at A, but because of the people they have to bounce around and take a much more circuitous path to get out of the room. This means that despite remaining at A the entire time, it took them longer to get out of the full room than the empty room.
The same is true with light. The light doesn’t ‘slow down’ in water. The light’s still moving at c. However, water is much more dense than air or a vacuum, so in order to make it through the water, the light has to take a much more circuitous path. Despite never changing speed, we as an outside observe perceive the light taking more time to cross through the same distance as a difference in speed, rather than what it actually is: the distance having changed.