DouNH;
They were checking that your registration and boating certificate were valid most likely.
Sounds to me like you did pretty good! OK, so the marine patrol stopped you for what they thought was an infraction...maybe you didn't think you did anything wrong but they did, no big deal...many of us have "rolled through the stop sign" in the past!
Anyway, they did their required safety check which you obviously passed with flying colors (congrats!), gave you a simple warning, the kids got a couple of trinkets which I am sure they enjoyed and you were off on your way with no out of pocket expense. Everything and everyone respectful of each other...
You did good!
Dan