I think the big mistake made by most travel agents is that they assume that everybody have a lot of money! They are therefore inclined to advertise and promote the most expensive accommodation and dive packages available only.
You will therefore find that most serious divers, who want to do as much diving with their hard earned cash as possible, arrange their own dive trips.
So, if you want to be a successful dive travel agent, you must also give your clients the choice of budget accommodation at reputable dive operators offering discount dive packages.