The prop on many single inboard engine boats turns clockwise(CW/reverse/right hand rotation) to help trim the boat.
When a boat is operated by only one person sitting on the starboard (right) side, the operators weight creates a list to the right. If the prop turns CCW (std automotive rotation), the torque it produces will also create a list to the right. So by turning the prop CW, it's torque now creates a list to the left, which will help counter the list on the right caused by the drivers weight. Hopefully the two are near equal so the boat will trim up level.
When more than one person is in the boat, they can shift their sitting positions to help with trim, one person doesn't have that possibility.

A bit of trivia, Chrysler Marine V8s didn't use the AVS's, almost all used a AFB's modified for marine use.