Yes, you will have to put down the deposit. As CP said; agents can only keep a complimentary hold on the cabin for about week.
Cancel the booking and when you have the money for the deposit then make your reservation. I doubt a cruise that far out will sell out by the time you have the deposit. If it is a "specific" cabin you want then try hard to get the deposit ASAP.
I also agree with Kuki. Just because an agent has an online presence they can also be a very good cruise agent. Cruise Planners and American Express Travel are examples. Yes, there are some places that have a "Gnome" and you may well get someone who knows nothing about cruising and fare structures.
Many years ago I used the Gnome site to book a cruise because of the incentives they were offering. I was actually quite pleased with the service of the representative.