Sea cave vs Sea arch
I don't know if there is a definition of the difference.  Sea caves are typically erosional features not solutional and do not have much length like typical limestone caves so to restrict them to only being a cave based on a totally dark section would force the reclassification of many sea caves to something else.  

Typically an arch has an opening diameter greater than its length.  In our parts, Pacific Northwest North America, a sea cave can be just about any opening in the cliff where there is a section that is continuously shaded.  The cave can have multiple entrances.