Jay,
You should go for a steering servo with metal gears and a bit more torque.
The Hitec Hs 645MG is an example, that would probably be regarded as a minimum starting point. It has around 9kgs (133 oz) of torque on 6 volts.
The reason for stronger gears and more torque is the servo needs to control more weight and is subject to more stress than on a mono. Especially, when the boat flips and goes into the water backwards
Some people like to use quarter scale servos for steering on their Mod boats because of the stronger gears.