If you are going to be on a boat for more than a few days you do not need any bow thruster. You will and should learn how to steer and control the boat. You should never find and do not need anything like a bow thruster on a twin engine bareboat charter cat. The wide beam separating the twin engines will give you all the control you need. The only exception to any of that would be very crowded and tight slips that you should not see regularly on a BVI charter. With more than six weeks if you will have the time to make a second pass or two at a mooring ball. Just take it slow until you are comfortable steering and stopping the boat.