I'd use plastic. Maybe recycle various model kits, which is a time-honored tradition for making sci-fi models. You might look at model train kits for pre-made building shapes. To light the buildings, you can hot glue LEDs into them (LED are waterproof and don't heat up), then make sure that the wiring is all sealed against electricity leaks (again, more hot glue everywhere). Plastic bottles could also make good building shapes, and the cut-off bottoms would look very sci-fi -- assuming that's what you're going for. Random plastic objects in general (thrift shopping would be good), like old toys, appliances, food containers, and knick-knacks can be repurposed for architecture.

When do I get to see some of your movies?

Underwater is one of the things that is fairly easy to simulate using 3D applications. Volumetric lighting and bubble particle systems are fairly standard in most applications.

I guess it depends on your shots and the need for specific buildings or architecture.

Ruins are afordable and abundantly available.
www.daz3d.com/i.x/search/...06bbcce3a7b/