This helps one find ships in waves going up and down (5)
I believe the answer is:
radar
This clue is a double definition.
'this helps one find ships in waves' is the first definition.
(system for detecting ships with radio waves)
'going up and down' is the second definition.
(Other definitions for radar that I've seen before include "Locator of plane" , "Radio detection system" , "Ranging system" , "Electronic rangefinder" , "System that detects and locates distant objects" .)