hsiaoyunh“What's the difference”
The difference is in what is called "aspect" -- the way you look at a situation -- the way you conceptualize its occurrence in time.
In the first example, you are thinking about what you saw as a single occasion or event. You are presenting your listener with a still photo of the fishing event.
In the second example, you are thinking about what you saw as an on-going activity. You are presenting your listener with a short movie of the fishing activity.
CJ