Any time you are breathing pressurized air underwater there is risk. There is the potential for a fatal embolism even at 15-20 foot depths. (Admittedly low, and you have to do something stupid on top of simply breathing from a scuba outfit - like holding your breath during ascent.) I took an underwater operations course in college as a technical elective for my degree. They didn’t gloss over the risks like they do in diving schools.
As for the value of wrecks, their main value is the knowledge of the period from which they sank. It really is a time capsule. I went into that in some length in Texas Shipwrecks, a book which involved a lot of discussion of marine archaeology.