First, you should NEVER book a cruise with an online site. If you need assistance, you'll get someone who is not a certified travel agent
I'll have to disagree with Pete on this one. MANY certified travel agents have BOTH an online prescence, as well as a brick and mortar office. As a matter of fact, many Cruise Planners Travel Agents would qualify. They have web sites, with booking engines available.
Just because they are "online" is no kind of disqualifier at all.
However, I do agree, you're much better off checking the qualifications of the person you are attempting to deal with, to find out if they are an agent, or an order taker in a cubicle somewhere.