Do I need to be a good swimmer?

Physical: For safety, all students complete a brief scuba medical questionnaire that asks about medical conditions that could be a problem while diving. If none of these apply, you sign the form and you’re ready to start. If any of these apply to you, as a safety precaution your physician must assess the condition as it relates to diving and sign a medical form that confirms that you’re fit to dive.

Water skills: Before completing the PADI Open Water Diver Course, your instructor will have you demonstrate basic water skill comfort by having you: 
* swim 200 meters/yards (or 300 meters/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 that you want. 
* You don't have to be a great swimmer, but you do need to be comfortable in the water and physically fit enough to complete these water skills.

About Physical Challenges: 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. Individuals 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 Amber Waves for more information.

Is it hard to breathe underwater?

PADI has some great information about breathing underwater. Check out their page located at http://www.padi.com/scuba/scuba-diving-guide/start-scuba-diving/breathing-underwater/default.aspx.