How do you distinguish sea cave from sea arch? If I want to classify a crack of sea cliff as sea cave, has it to have a dead end? Would you call a geological formation as a sea cave even a kayak can pass through?
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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.
I would say they both require having rock over top of the water - so not just a water passage between two cliffs. For land caves, the distinction between a cave, a rock shelter, and an overhang generally depends on whether you can in get beyond view of the entrance, but for sea caves, that generally isn't how they are named. But to be considered a cave you would have to be able to enter and it should be longer than it is wide.
An arch will have an opening to daylight on both sides of the arch. A sea cave will have a dead end. There are many cases in conventional caves where there are multiple entrances but I think that is exceedingly rare in erosional sea caves. Some of the distinction then could be just semantics and how someone decided to name it. If it were to occur, the distinction would probably be made on the distance between the entrances - can you see daylight from the other entrance,