In my 1/8th scale unlimited hydroplane(shown in my avatar), I use the Futaba S9156 for the rudder. The write up on it is:

• Ideal for any heavy duty loads such as control surfaces on 1/3 scale and larger airplanes and jets as well as boat rudders
• Metal gear train eliminates backlash and slop
• Aluminum center case helps to dissipate heat
• Torque: 272 oz/in (19.6 kg/cm) @ 4.8V 340 oz/in (24.5 kg/cm) @ 6V
• Speed .21 sec/60° @ 4.8V .17 sec/60° @6.0V
• Dimensions: 1.6 x 0.8 x 1.4" (40 x 20 x 37mm)
• Weight: 2.2oz (63g)

I've not had an issue turning the boat using one powered by a 4.8volt nicad pack. It is also standard sized so it fits pretty much all commercially made servo mounts. It should be more than enough to turn your boat though it is spendy at $130