Meeting the Past, the submerged walls

The constructions occur at depths from 2 to at least 4 m. They were built when the sea level was much lower than the one of today, at a time when Despotiko, Tsimintiri and Antiparos were connected by an Isthmus. 

Archaeology is not my field... but those submerged constructions just intrigue me, arousing curiosity!

1. Circular (cylindrical) wall 

The construction is about 1.20 m in diameter and 60-70 cm high, located at 2 – 2.5 m depth... It was already observed by James Theodore Bent in 1885, using a sort of can with a glass bottom (from a sponge fisherman) ... thank you David F.W. for the reference!

It could be a remain of a water well or reservoir, or a particular place where a fire was kept (sort of small lighthouse) to indicate a particular site or passage?
 J. T. Bent considered it was an oven. 

2. Straight walls

One straight wall 'intact' and a few others in pieces. The intact wall is orientated towards Despotiko. Several remains of damaged straight walls are dispersed in the vicinity. 

Did these walls simply delimit land (like those of today on the island)? 
Are they located along a ancient road/path/passage, for example, along the isthmus that connected Despotiko, Tsimintiri, and Antiparos?
The intact wall seems also to be aligned with a valley… did it border a river?

3. Straight small channels

Is this structure an opened canalisation ? Edges seemingly collapsed but the bottom of the channel and its edge are still visible. It seems to have the same orientation than the straight intact wall.

4. Bases of square or circular constructions

These could be remains of small houses or of graves or…? A small square house was reported by J.T. Bent in 1885.