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.
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