Yes, you could add a depth sensor for not much money in overall cost of a commercial passenger aircraft (maybe USD10,000) BUT, and this is the big BUT.... you still need a direct "line of sight" to the FDR to be able to pick the signal up. If it's stuck in an undersea crevass etc, you will never detect it (see earlier posts).
Regarding recent new point raised: I used to work for a company that made automotive crash sensors. Typically these trigger at 14 to 16g. They had to withstand 8g without triggering; every one we made was tested for performance.
Edit: Thinking about it, the RovHomer calculates distance and angle IIRC, so it automatically calculates the FDR depth (or the control software does)