The depth of 600 ft [approx] is well within the capacity of Saturation Divers using a Mixed Gas System, most likely Hydreliox or similar.
Various ROV’s maybe able to undertake some work, however as the FDR/CVR devices may have required leverage, cutting or other manipulation to free them, it’s more likely that Sat Divers were used.
Saturation Divers would also make the relocation of the badly damaged airframe onto/into the retrieval framework more easy, together with the more sensitive handling of our colleagues and passengers.
Just my private view.