And double bah!
Here's an idea no less feasible than slime tanks lining the old cavity walls:
How about so-called "scientists" genetically engineer open-air, photosynthetic coral-building organisms. You could smear your walls with them in a nutrient paste and, once the colonies had established themselves, start building up layers of coral on your walls.
Much nicer than pebble-dash, greener than stucco and what other finish can you apply to your house that will eventually add another floor and a garden shed to your manor, with no effort on your part? Need a granny flat? just wait about five years and the old bat can move in with no expense spent over and above the original seeding (which you'd do out of pure eco-mindedness and to insulate the house against the bitter global-warming-induced winters).
Instead of sinking ships as reefs you could tow 'em into position and anchor them, give 'em a good spray of Stevie's Patent Coral Goo and leave them to turn into real coral, capsize with the weight, sink and become real submarine nature preserves.
Why, I should get a Nobel Prize for Thinking Up Worthwhile Stuff For Scientists To Do just for the idea itself!