Don't worry Supaboat, we're raring to help you.
If going for what sounds like a displacement hull (Tug, Merchant chip, Tanker, Support ship etc) rather than a speedy affair like a cabin cruiser or a fast electric boat, then the stresses put on the rudder and propeller are less so you do not need such powerful servos or motor.
The size you quote also helps with making it easier, and potentially cheaper to build. A Tanker/merchantmen model say six feet long will weigh up to seventy-100 pounds (?) and need a motor that can stop and start that with matching ESC, and a servo to keep the rudder in the path of the prop when turning.
So, you should be able to go for a lower amp ESC and smaller motor in the '200' range and a standard servo. You could run it on 6 volts. All this should save you money.