The "Cove of the Sower" is a natural amphitheatre on the western side of the sea of Galilee, and is the site traditionally identified with this episode in Mark. I am not sure whether any other natural amphitheatre exists on the shore on that body of water, but at least one does.
The familiar design of the amphitheatre was well-known in Hellenistic cities, and there was at least one in the Galilee (at Sepphoris), and most likely one in each of the Decapolis cities. I would speculate that the development of the amphitheatre design had its origins in the existence of natural amphitheatres, both seaside and well away from the water.
I am not sure whether we have any literary examples of natural amphitheatres being used by speakers when addressing large crowds away from a public venue in a major town. Even if we did, that would not solve the historical question of whether Jesus used such a natural amphitheatre (which is known to exist) to address large crowds from a boat (which is a detail known only from - and possibly created by - Mark).
The addition of literary embellishments (such as the pillow for Jesus' head in Mark 4:38 or the green grass on which the multitudes sat in Mark 6:39) is simply the sign of a skilled story-teller and in no way implies a "factoid" preserved from some original event.
In terms of historical inquiry, we seem to be caught between the macro-, meso- and micro-historical levels:
- MACRO - realities in place for millennia, and changing so slowly as to be almost imperceptible (climate, geography, human nature, etc)
- MESO - realities in place for centuries but changing as time passes: culture, economics, literature, politics, religion and technology
- MICRO - particular objects, persons and events
The physical reality of natural amphitheatres (at least one) at the Sea of Galilee is an example of a MACRO historical element
The technological development of artificial amphitheatres (or timber fishing boast, for that matter), and the political development of Hellenistic cities with certain preferred public infrastructure (amphitheatre, gymnasium, stadium, etc), together with the cultural developments of writing, hero stories, etc and the religious development of early Christianity generating its own sacred texts, are all examples of the MESO historical level - and, incidentally, the level at which historiography usually operates
The question of whether Jesus existed, or whether he said a particular statement, or whether he performed a particular action (such as standing in the boat owned by local fishers who were followers/disciples) are all examples of the MICRO
My own take on 196 From the Boat is that such things were typical of leaders addressing large crowds (e.g., generals speaking to their troops) out of doors, but that the particular instance in Mark 4 (which I take to be the source of the variants in Matthew and Luke) is a literary creation by the gospel writer to portray Jesus as a charismatic figure who attracted and address large crowds out in the open fields and away from the towns/cities.
What I find problematic is not the use of a natural amphitheatre but the claims that Jesus was drawing large crowds, as this would be a political development of great interest to Herodian rulers of Galilee and the Roman occupation forces more generally. We know what happened to charismatic leaders who gathered large crowds out in the "desert" places - they were killed by the military and their supporters were dispersed. Of course, that is exactly what happens in Mark as a whole, but the plot is rather more complex and the murder/dispersement is delayed until chapters 14 and 15.