Some swimming ability is required. You need to have basic swim skills and be able to comfortably maintain yourself in the water. Float and tread water for 10 minutes, again using any methods you want.

With Padi you need to complete a set order while SSI and SDI allows slightly more flexibility.
Other than this the courses are more or less similar: all offer online material and cards, are accepted all over the world and maintain high standarts of practice. But in the end its the instructor your experience depends on… with Oceanic Divers you are in good hands.