CED Investigative Technologies / ABYC boat boarding ladder test showed longer ladders made boarding boats safer by keeping people more stable, and away from the propeller.
Aspects of the Propeller Safety Debate chart
Houseboat Propeller Regulation Proposed and Withdrawn by USCG: Analysis by PGIC
History of Boat Propeller Safety
Mitigating Boat Propeller Injuries & Fatalities Chart