When I shot film, I always carried two 2x neutral density filters. With slow slide films, I generally aim for 2 to 4 second exposures when photographing waterfalls, waterflows, etc. These filters are stackable to get different degrees of light reduction.
This is even more important now since digital equipment won't go down to ISO 25.
Shot on Kodachrome 64
Canon A-1
FD 24mm f/2.8
Exposure unrecorded
1989