I think it's important to note that evolution is really just the change of the availability and distribution of heritable traits in a population over time. To me, it seems fairly obvious that there are slow and fast components that can effect this, and therefore evolution plays out at multiple timescales simultaneously.

Your example is one of pretty fast restriction of traits: The trait of not being able to swim pretty quickly disappeared. However, how long did it take for the ability to swim to be present in the population in the first place? How long will it take for individuals to exist without the ability to swim after the event?