Whenever we cruise, I always call or online book directly with the cruise line that we're sailing with. This keeps you in control of your booking--if you book with a travel agent, that travel agent owns/controls the booking and sometimes the cruise line will only deal with the agent that booked the travel. This may mean that you need to go through the agent if you need to make changes or have concerns about your booking.

I believe that some/most/all? cruise lines have some sort of Personal Vacation Planner (PVP) that you can book with. This will act like a travel agent who is (I believe) a paid employee of that line. It's their job that you have an excellent experience with the booking experience and to answer all of your questions. As you develop a relationship with this PVP, they will get to know your preferences and they will watch out for you.