From what I've seen, a degree in business and/or marketing would be a huge plus. I think a lot of dive shops suffer because the people running them are divers and not businessmen.
I also think one of the big challenges is adequate capitalization. Many shops I've been in have very little inventory, because of the investment required to collect same. I think adequate inventory is key to doing retail sales in the internet age -- If I have to order it from the shop at full retail, or I can order it from somebody else at a big discount, which am I going to do?
And finally, almost all the shops I've ever been in are mainly run by the owner. In other words, he's the guy behind the counter making sales and answering the phone. That makes it difficult for him to run classes as well (and difficult for him to dive sometimes, too).
Well on the other hand I know some Dive Shops which are run by businessmen. Surely professional, but you have the feeling is about money. Professional perfectly clean and steril, perfect dive tours which are suitable for everyone..... Always a DM close to you nursing you. But no fun anymore.....