Hi Tom - It's ScubaFit.
I just finished reading your answers again. Your lifestyle information is helpful. Many times it's simply a matter of scheduling. Sometimes it's a matter of fatique or burnout. And, of course we have already covered lack of interest or enjoyment of exercise.
Here are my thoughts:
1. It should NOT bother you that you get your exercise through sport activity such as diving. Embrace it!
2. You are what I would consider ScubaFit based on the description of your shore diving activity. If you weren't ScubaFit, you would not be able to dive in this manner. Dive days are workout days for you! In addition, your previous cycling has contributed to a longtime fitness level.
3. If you want to add something more to what you're doing, my recommendation is additional activity two times each week. This activity could be (if you need a motivating reason) to enhance your diving even more. If you're like me, you want to dive forever and remain injury free. Other motivating reasons can be health concerns/clinical conditions such as high blood pressure, diabetes, etc. If you have any of these conditions, your option to exercise or not has greater benefits/consequences.
4. There is a full spectrum of exercise to choose from. Perhaps a "mind captivating" activity would be a good place to start. Your fitness goals should include both aerobic (heart and lungs) and anaerobic exercise (muscles). While I personally believe that nothing can replace weight training in itself - picking up a rock and setting it down again is also weight training. Other anaerobic activities (involving stop-and-start and/or balllistic muscle response with an adequate oxygen pathway) are bowling, tennis, racquetball, softball, baseball, volleyball, golf, some dancing and group exercise. You will accomplish your fitness goals with any of these activities and develop strength endurance that enhances your diving. Aerobic activities that will accomplish some strength with more cardiovascular are swimming, running and cycling.
5. I recommend you take advantage of social opportunities to engage in these activities. Family time, church and/or dive club picnics, corporate events, etc. Supplement exercise with purpose when life doesn't offer the minimum of four hours or sessions of activitiy each week. For example, if you don't dive next week, make sure you do something else, such as golf with a business associate, help a friend move or go hiking during a family camp out.
In closing. My impression of you is that you are someone who is usually good at both the plan and the process. Exercise doesn't have be done at the same intensity in life that you do everything else to be a positive. You have a full schedule and a full life. Enjoy it. .
Hope this helps.
ScubaFit