The dead man has been named by Pattaya police as Thomas David Grieve, 57. Seriously injured and in hospital is his companion on this afternoon's flight, Patrick Esier, 31.
It is believed the men were scattering cremation ashes and floral petals into the water when the aircraft crashed.
The man who was seriously injured is understood to have been representing friends and family who were in two speedboats below the aircraft when it crashed.
The microlight plunged off Pattaya when the small engine on the two-seater failed, witnesses reported.
The men performed two circuits, then crashed into the sea on the third circuit, local reports said.
Paramedics said that Mr Grieve suffered broken ribs that the ribs caused internal damage that resulted in his death while he was being rushed to hospital.
It is not known who was being mourned at the time.
Mr Esier is being treated in Pattaya Memorial Hospital.
Microlights, also known as ultralights, are popular recreational aircraft in many countries.
The crash came about 2.30pm off Bali Hai Pier in Pattaya.