Good Samaritan laws are definitely applicable in this case and will protect you.
Just as an aside, let us not forget that a designated "buddy" is not a good Samaritan, and most likely not a bystander under rescue analysis. This of course doesn't apply if you're just diving off the same boat with someone else.
I'm pretty sure the DM/Rescue Diver cert issue has been explored in-depth previously, but iirc, once you initiate rescue, you would be held to the level of competence of a rescue diver and not an ordinary person? I'm sure a litigator will step in to correct me if I'm wrong.