You can book ahead of time if you feel you need to do this but you'll be paying full price. On every cruise I've been on they have specials for "couples" massages which bring the cost down $50 or so which you can then turn around and leave a nice tip for the massuse. I wouldn't worry about them filling up for the simple reason, again, they are specials on every cruise! Enjoy and Happy Anniversary.
You've already gotten some great advice. Don't know why your agent said you couldn't reserve it ahead of time because you can. But as was stated by others, you may wish to wait until you get onboard and see what specials are offered. I suggest as soon as you get on the ship that you go there, do a tour, and talk to them about when they might be offering the specials during your cruise. And as was also pointed out, port days will often find the best deals since most passengers leave the ship and they have nothing to do. Sea days find them busy, so you won't see specials then.
However you end up doing it, you're in for a great treat!
__________________ 46 Cruises & Counting! Favorites: Paul Gauguin to Tahiti: Uniworld River Cruises in Europe; any of the Celebrity Solstice-class ships; Holland America for 12-nights in the Baltics & Russia; RCCL for 14-night Greek Isles, Turkey, & Croatia; Holland America for 14-day Alaska cruisetour; 10-night Canada/New England cruise; 21 days in Hawaii including a 7-night NCL cruise; Oceania for 25 days in Asia; & 3 months touring Europe by train. And many days spent in all-inclusive resorts!