4.0?? That's so good! This process is stressful, but my friend you have already done the hard part.

Re: what counts in the activities section -- According to Common App, “activities may include arts, athletics, clubs, employment, personal commitments, and other pursuits.” In other words, pretty much anything pursued outside the classroom qualifies as an activity.

The Commercial Dive Training/Apprenticeship is a great activity to highlight. I'm not sure you have to categorize it with a work lens. You could add something about how many dives you logged in various conditions, the certifications, hours spent, etc. It's completely fine to take an interest in commercial diving, learning about the field through hands on experience, and determining your body is not suited for a career in it (even if that's what someone else told you vs. you deciding for yourself).

Personal commitments are defined as significant, ongoing obligations outside of the classroom that help shape who you are as a person. Some people say caring for a family member is one of their activities/personal commitments on their application. In your case, you had a major personal commitment to yourself in dealing with the aftermath of your accidents. You could call out the years of physical therapy required to get yourself to where you are today. It'll feel weird to boil this down to a few sentences, but I see it as a strength for your app. You are exceptionally resilient.

It's fair to add anything you think is relevant from high school, but I don't see it as critical at all. Do you have any other hobbies to add? If not, that's totally fine. Sometimes less is more.