Divers Direct is about1/2 clothing and water sports stuff IIRC.
The fact that you have 3 big sporting goods chains in trouble at the same time is probably not a coincidence. Dick's missed earnings, SC shuting down and Sports Authority just deciding to go into liquidation is not a sign of health for outdoor sports in general.
I've done some research into this for school. The major reasons are that people prefer specialized stores or the ever-increasing online market. It's odd that Sportcheck (comparable to Dick's) in Canada has managed to even expand. But, I for one would rather go to Nike for running shoes, Golf Town for clubs, etc.
I don't think that putting a dive store in a mall would work. People go to malls to shop, usually leisurely; not to sign up for an [extreme] activity. I couldn't foresee many people going to a mall to buy shoes, then thinking "hey, maybe I'll sign up for scuba lessons" on the way to the food court. If people are already divers, they are probably more than willing to drive to a specialized store, in an obscure area, to purchase products.
There may be some indirect run-off of people subliminally becoming interested because they walk by a dive shop a hundred times, but that would be marginal. I would bet that the store would be pushed towards selling snorkels, masks, flippers and even bathing suits, rather than very specialized gear. There was a dive shop near my house, which is not in a mall, but is in a very urban area. They now almost solely sell wakeboards, tubes, water skis and things of that nature. I haven't been there in a while, but I don't think there's any signs of scuba gear.