Gerd, Thankyou for your detailed reply.
I can see how rectangular prisms of varying lengths, width and height would be made using this method. Each such module could be quite large, though there would be practical limits to be applied. And real estate I think is most likely restricted to be one on each module, rather than across modules.
I should have thought of filling them with PET Plastic bottles (as in Richard Sowa islands), though the government here has just put a price on these by way of a refund.
I would think that to create a large island that could be sufficiently resilient in the open ocean, there would be big modules which would be tightly lashed together along a common vertical wall. The lashing would be tire strips that thread between the tires to hold two rods tight together. Assuming a new module floated out would already be filled with PET bottles, I can see that the edges of the modules can be tightly lashed together but not the inside of the vertical walls. This is not likely to be sufficient.
There are three ways I can see to get around this. One is to put one metre cubes between the two vertical walls and lash these to both walls, and probably to each other. Put the PET bottles into the cube after the lower sections are lashed, connect the top and then also lash the top horizontal edges. Even with the cube empty, the vertical walls may need pushing apart while lowering each cube down into position.
The point is an island could be built up in a modular fashion. The highest modules in the middle and gradually lowering to the outside.
If the waves mostly come from one direction. then have smaller modules on the outside facing the waves, with loose coupling between them so they dissipate the waves without affecting movement of the middle of the island so acting as a breakwater (I think you said this). These modules would be unsuitable for walking on normally due to the instability.
On the leeward side of the island, the modules would be tightly lashed together, and eventually forming a bay for boats possibly. Being solid together, people can walk between the modules and down to the boats. Also on the tightly lashed together modules, soil and plants besides the lightweight buildings and pathways could be covered on top.
Between the modules, mooring lines could be placed. If cubes used between the walls then leave a one metre square hole say between each as required. In keeping with the theme, the mooring lines would be made of tire strips connected together (as proposed by Spark).
I think the point is: Although lots of work would be required, there is certainly plenty of potential in this!