Scuba Refreshers typically involve some book work and in-pool skills work. Depending on how long it’s been since the student was diving last, how much they remember from their class, their comfort level, etc. scuba reviews or refreshes can last between 2 to 3 hours.
Yet I wonder, would anyone want just a theoretical or book review without the pool / skills work? Perhaps if you intend to go from Open Water diver to Advanced, you might want to review some book skills. Who else might?
If there were enough users who just wanted an online refresher, review or as PADI calls it a “Tune-up” it might be easy and cheap to offer courses online. PADI already offers their Tune-up as an eLearning system which you can find here but it’s $55. With the eLearning course you still need to do the in-pool portion or practical phase which (I’m assuming) will cost you additional monies.
It might be cool to try creating a free or cheap Scuba refresher online through uDemy.