The circuit for this project is quite simple, just connect the Vout pad on Circuit Playground Express (CPX) to the servo's red wire, the A1 pad to the yellow wire, and the GND pad to the brown wire.
The alligator clip-to-male jumper wire connectors are great for connecting servo motors to CPX.
Circuit Playground Express can use a variety of power sources, anywhere between 3V-5V is safe. These compact 400mAh lithium ion polymer batteries are excellent for powering small projects like this.
A set of 3 AAA batteries also provides ample power, if that option is more convenient. If you're using a AAAx3 battery pack, check that the switch on the battery pack is set to "ON" when you want to power the boat.