BoatUS Foundation has a collection of state-specific free boating safety courses that you can take online. These courses are designed specifically for your state and most of them are recognized by the U.S. Coast Guard and approved by NASBLA. There are only a few states at the bottom that are not approved and only intended as a refresher or study guide for an in-person exam.
If you live in one of the green, recognized states, then you'll not only be able to take the course, but after you complete the course you'll be issued a printable copy of your certification that you can print & use immediately.