With a scale displacement model, if you know the horsepower of the real thing, you can scale that. Multiply by 750 to turn HP into Watts, divide that by the cube of the scale, then multiply by 2 to make up for motor and model inefficiencies.
Knowing the watts gives the current at any voltage, but remember that the power demanded by the motor is determined by the load imposed by the prop.
Size of battery. Generally as much as will fit (both the space it takes and the weight) to ensure that it will be able to supply the current needed for as long as it is wanted. Different boats have different power requirements, both plain basic power and the speed of rotation (RPM) that the motor can actually do.
Matching the motor and prop is important. Fast boat, small prop being turned very fast, slow but powerful, large prop turning more slowly. Much repeated rule of thumb is "Prop diameter no more that the motor can, prop should have fewer blades than the motor has poles". Thats for direct drive brushed motors. Gearing effectively increases both effective diameter and nmber of poles. Going brushless is a whole different world.