Wave pole

From Glossary of Meteorology

wave pole

(Also called wave staff.) A device for measuring sea surface waves.

It consists of a weighted pole below which a disk is suspended at sufficient depth for the wave motion associated with deep-water waves to be negligible. The pole will then remain nearly as if anchored to the bottom, and wave height and period can be ascertained by observing or recording the length of the pole that extends above the surface.
See wave recorder;
compare tide gauge.