Could I use a relay shield to control a servo?

I'm tryingt make a rov but I want to make it like a submarine where the back of it has like what they have on airplanes to control the y and the x(the flaps)

Just looking at the words in the subject heading - NO. A servo needs pulses, too fast for a relay.

Good luck withthe project as described in your post. Just getting the thing watertight and have neutral boyancy so a elevator/rudder will work is going to be fun.