I've never built the Crockett but, the way I see it, you really have two choices:
1) Install the servo under the deck house, which I believe is supposed to be removable for interior access, then run a pushrod to the gun mount, installing the gun mount and pushrod before gluing the deck down. With this option, the servo will be accessible but the bottom of the gun mount won't
2) Install the gun mount in a removable section of deck with all the mechanical and radio gear needed for operation attached. If you do this option, I would use the aft side of the breakwater, just ahead of the gun mount, as the forward end of the removable section as it's protected from water coming over the bow at that location. The rear and sides of the removable deck would be your call, depending on how much access room you feel you need and how small of servo you install.
Either choice will work and choices like these are what model builders have been making since models first hit the water. It's really up to you as to which option, or something else you may like better, that you go with. I'm presently trying to work out the radio installation on some 1/8 scale hydroplanes I'm building and have come to the point where I have to figure out where to put the rudder servo; in the radio box at the front of the boat with a 20+ inch pushrod or under the deck at the rear of the boat with a fairly short pushrod and then figure out how to access it, from the inside through a bulkhead or through a hatch in the deck. Decisions, decisions, decisions