According to Official Guinness Records,
The largest scuba diving lesson was attended by 2,465 people and was achieved in Malalayang beach, Manado, Indonesia, during an event organised by the Indonesian Navy, on 16 August 2009. Some basic scuba diving techniques like the mask recovery were performed during the scuba diving lesson at a depth of about 10-15 meters.
For a complete list of 2009 records, please visit 2009 Guinness Records in Indonesia.