...but what kinda comission do the shops actually make on scuba gear??
This really depends upon the shop and the item. When I owned an LDS, I sold tanks as a lost leader at cost. Other major pieces of kit, I took my cost and doubled it, so a $450 B.C. cost me $225. Higher mark-ups were made on accessory type items. Filling cylinders was break even. I had my air analyzed independently every 3 months and posted the results.
I made nothing during training, as my courses were 50 hours, so to be competitive I lost what could have been made, but ethically I didn't want to let someone out the door half-trained. If I ran twice the number of courses, I would have greatly benefited in the sales department (thus the genius of the short PADI programs).
I made money on dive charters, especially live-aboard charters on the weekend. The business was marginally profitable, but a lot of work (read time) for the return.