Question

Waves are observed passing under a dock. Wave crests are 8.0 meters apart. The time for a complete wave to pass by is 4.0 seconds. The markings on the post submerged in water indicate that the water level fluctuates from a trough at 6.0 meters to a crest at 9.0 meters. What is the frequency of the wave?

Answer 1

To answer that question, we don't care what the highest and lowest
levels of the wave are, or how far apart they are.  We only need to be
able to identify the highest point on the wave, and keep track of how
often those pass by us.

You said it takes 4 seconds for a complete wave to pass by.
Through the sheer power of intellect, I'm able to take that information
and calculate that  1/4  of the wave passes by in 1 second.

There's your frequency . . .  1/4 per second, or  0.25 Hz.