I have a current CPR certification. You have to renew every year.
However, keep in mind that CPR is only really effective if there is a chance that trained medical team can get to the person in about 10 minutes.
This is not realistically going to happen in any boating situation unless you are anchored next to the CG station.
If you are serious about it, you should buy one of the portable defillbration units as knowing CPR is just something to do in first few minutes while someone gets the defib machine.
Red Cross CPR Website
"Step one: Early access to care (calling 9-1-1 or another emergency number)
Step two: Early cardiopulmonary resuscitation (CPR)
Step three: Early defibrillation
Step four: Early advanced cardiac life support, as needed
The third step, delivering an electrical shock to the heart, which is known as defibrillation, is recognized as the most critical step in restoring cardiac rhythm and resuscitating a victim of SCA."