Does ground clutter exist over the ocean?
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)