A Spanish research vessel that investigates marine ecosystems has been abruptly diverted from its usual task to take on a new job: Helping in the increasingly desperate search for the missing from Spain’s floods.

The 24 crew members aboard the Ramón Margalef were preparing Friday to use its sensors and submersible robot to map an offshore area of 36 square kilometers — the equivalent of more than 5,000 soccer fields — to see if they can locate vehicles that last week’s catastrophic…