I can't recall how she went down; even keel, bow down or stern down?
That'll have a big impact on what you can do.
Best option is if she went bow down, even on a relatively even keel, because that way you can model her with her stern & some of her superstructure out of the water (you should take latex casts first, for making resin copies) as she's going down & then have the sea floor section using the front 2/3rds with her stern "covered" with volcanic debris. The resin copied superstructure can then be fouled with a thin coating of marine growth & silt, etc.