Sam,
I believe the boat already comes with a motor, shaft & tube and propeller.
For basic functions (speed control and steering) you only need a two channel R/C set but you might as well get a 4 channel 2.4ghz one as they only cost marginally more and give the opportunity to add extra functions. Any standard servo should work OK for the rudder and you will need a speed controller.
Have a browse on the Howes model website as they sell everything you might need:
**LINK**
For the main drive battery a 7.2v pack would probably be OK:
**LINK**
You will also need a charger of course and Component shop will sell you a suitable one to match your battery pack.
Colin