Does ground clutter exist over the ocean? Ground clutter is a radar term that basically means a bunch of noise, or cluttered returns, that occurs when you point the radar down at the ground (like for a radar patrolling aircraft). The terrain reflects radar and you get a big blur.

Today, ground clutter is not a problem because we have great computers that can filter out the static ground returns, and only show the moving returns. However, I want to ask if ground clutter needs to be filtered when over the ocean.

The ocean is pretty flat, but always has at least some small waves. Do small waves produce any ground clutter? In other words, does ground clutter exist over the ocean during calm seas?
Yes, ground clutter does exist over the ocean. It depends on the smoothness of the water surface, radio frequency, antenna characteristics and atmospheric properties whether the radar will actually receive reflections from the sea surface. 

A perfectly smooth sea surface will not generate clutter, but the combination of small waves and a strong inversion layer (temperature getting higher with altitude) will cause ground reflections, even for surface based radars looking above the horizon. The inversion layer will cause super refraction - the radio wave bends towards the ground / sea - and will cause a reflection off the waves which is received by the radar. 

In an extreme inversion layer a so called atmospheric duct may form in which the radio signal makes multiple bounces of the sea. At each bounce part of the energy is reflected, resulting in multiple ground clutters at regular distance intervals. 

Ground clutter is not always perceived as a problem. In some cases its absence may help to identify objects of interest. For example, in weather radar the absence of ground clutter behind a cloud indicates that the cloud has absorbed all the radio energy which is an indication of its severity. 
 

source: Airbus Flight Operations Briefing Notes, Adverse Weather Operations (PDF)