Operator was on plane, when the motor trimmed up on its own.  Operator stopped the boat to see what happened, tried to trim the motor down and it wouldn''t move.  Started idling back toward the ramp when occupant said there was water coming over the back.  The transom went under water and the boat rolled starboard.  The boat capsized and all dumped in the water.  Two lifejackets were on the backs of the seats and a third was under the console.  Neither occupant was able to secure a PFD before going in the water.  
The seating arrangement was as follows: operator was the operator on the starboard side, occupant1 was in the middle, and occupant2 was on the port side.  All three were side by side.
Several hundreds of dollars of fishing equipment and personal items were lost.
DANDRIDGE