A scuba-diving instructor has drowned trying to save the life of another diver, who also died, on the Mornington Peninsula.

Leonie Hanson, 38, was diving with a 39-year-old man off Mornington Pier, near Schnapper Point Drive, on Friday morning.

A local alerted police after seeing the divers struggling in the water about 12.15pm as members of the public desperately tried to save the pair.

Ms Hanson was an Open Water diving instructor with Harbour Dive Australia in Mornington. The man was a less experienced diver, police said.