The man went under about 3:15 p.m. when he slipped out of his life jacket which apparently didn't fit properly, according to officials.
Divers from Central Pierce Fire & Rescue are now attempting to recover the man's body using sonar equipment.
The man lost his life jacket, while going under the water. It has been recovered but his body is still being sought.
The other two people who were innertubing are safe.
They were being pulled behind an 18-foot boat when the accident happened.
It is not clear whether the victim was conscious after flipping off the inner tube or if he or the boat hit anything.