I would think you would need environmental clearance from DoE and fisheries before proceeding with this idea. There are lots of issues with this, and the materials you plan to use will need to be carefully thought out.
You probably already know that many artificial reefs have been made in the US, especially around the Florida coast, using obsolete US army vehicles, ships etc. These have proved immensely successful in promoting wildlife and have become popular fishing marks for boat captains. The problem is that the "material " has to be stripped of anything which could pollute the water.
Early on, motor tyres were used but the consensus of opinion now is that tyres make very bad "material" as they break adrift and wash up on beaches eventually. You will find a lot of info on line about this.