Vessel 1 was being operated in a southbound direction when the operator changed direction and started to head north.  After making the turn, the inner-tube hit a large wave, or wake, which caused the inner-tube to go airborne.  2 of the three passengers on the inner-tube were thrown into the water.  One who did not have a PFD on, immediately began to struggle and appeared to have difficulty staying above water. 

The operator jumped into the water, not wearing a PFD, to assist the struggling passenger.  Both went under the water and did not resurface.
Webster                       