The coastguard received a call at about 14:00 GMT reporting a man in the water and an overturned boat at Ness Point.
Attempts were made to revive the man, but he later died. The coastguard said a search for two other people possibly on board would continue overnight.
Coastguard rescue co-ordinator Peter Byatt said the 18ft (5.5m) speedboat had been recovered.
Lowestoft and Southwold Coastguard, RNLI crews from Lowestoft, Southwold and Gorleston and an RAF search and rescue helicopter have all been involved in the search.
Humber coastguard had initially planned to suspend further searches until the morning.
A man has died and two others are feared to be missing after a speedboat overturned off the Suffolk coast.