Some swimming ability is required. You need to have basic swim skills and be able to comfortably maintain yourself in the water. Your PADI Instructor will assess this by having you:
Swim 200 metres/yards (or 300 metres/yards in mask, fins and snorkel). There is no time limit for this, and you may use any swimming strokes you want.
Float and tread water for 10 minutes, again using any methods you want.
Any individual who can meet the performance requirements of the course qualifies for certification. There are many adaptive techniques that allow individuals with physical challenges to meet these requirements. People with paraplegia, amputations and other challenges commonly earn the PADI Open Water Diver certification. Even individuals with more significant physical challenges participate in diving. Talk to your PADI Instructor at your local PADI Dive Center or Resort for more information.