Mike, this stuff is simple.
Set up a second corporation.
The second corporation sells the parts. It has its own books and accounting. It is nothing more or less than a retail store. (Heck, it could buy regs from LP and resell THOSE too!)
It is completely separate, legally and corporately, from your dive shop. It is therefore outside of the view of your insurance carrier for your dive shop.
If you want to get REAL crafty, sell the parts MAIL ORDER! Now you don't even have physical colocation! Require a written waiver (faxed is probably ok - check your state law) for all sales.
Find a handfull of guys willing to become corporate founders so that its not a "closely held" corporation under the law (the key is that you and your wife can't be the only stockholders) That solves the problem of the veil being pierced.
If you make a roaring success out of this, you might even make a small fortune on the parts sales - especially if this view of thinking "outside the box" spreads....
Just think Mike - you could solve your complaint and do something that would get you in "Who's Who" all at the same time
There is no such thing as "can't" Mike. There is "not profitable", and there is "I don't want think long or hard enough to solve that problem", but there is no such thing as "can't" when it comes to small business.
The problems you've presented here don't even register on the scale of true troubles...