Nev at Los Gigantes Diving Centre is fully legal, carries all the required safety equipment, including oxygen, and most important has the boat manned at all times when divers are down.
Can't comment on the other Centres cos I don't know.
Questions to ask Dive Centres:-
Are the Centre and boats legal?
What are Instructors qualifications? They must be DiveMaster or Instructor to do Try endash A- Dives, or lead qualified divers. MUST be Instructor level to do courses.
Is safety equipment including oxygen carried on all dives?
and most important (required by law) is the boat manned when divers are down?
Sadly many centres on Tenerife ignore the law and leave their boats unattended while divers are down, putting their staff and customers at risk.