The man suffered injuries to his upper torso and shoulder in the attack that happened on Friday.
He had been spearfishing with another man and a woman when the attack happened.
The victim, thought to be in his 50s or 60s, had blood on the front and back of his right shoulder, and also on his neck, the Palm Beach Post reported.
According to a witness on the scene, the man appeared to be "hurting bad" and the shark had "bitten right through his wetsuit".
The unidentified man was airlifted to St Mary's Medical Center's trauma unit.
"He was bleeding quite a bit," witness Peter Pinello told WPBF. "It looks like a shark had gotten him in the torso. You could see his wetsuit was embedded in it, and it looked like it took a good chunk of it out."
Palm Beach County Fire Rescue Battalion Chief John Vanek told WPBF that the victim was bitten on the head but was conscious when he was flown to the hospital.
"He had injuries sustained to the trunk of his body, so he's a trauma alert by our standards," Vanek said.
This is only the second shark bite in Florida this year, according to the director of the shark program at the University of Florida Museum for Natural History George Burgess.
Bull sharks are common near the coast, particularly in Florida, and prefer water less than 100ft deep with low visibility so they are not easily seen.
According to The Florida Museum of Natural History, there have been 323 shark attacks on humans in Florida from 2001-2014. Of those only three have been fatal.
The vast majority of shark attacks in the US occur in Florida.