What if I can not swim?
It is not a mandatory requirement to swim to get hired. It is highly unlikely you will ever have to swim, unless there is a boat drill and you will be asked to swim with a flotation device.
No matter how familiar you are with safety at sea, you will undergo numerous trainings not only your first time around, but every single time you return back onboard. Some training is available prior to joining the ship in some countries (China, the Philippines, for example), but you will be scheduled for basic training once onboard too. One of the perks is that some positions (in the Hotel department) have to drive a life-boat, so you can get a certificate for driving it.