An electronic system for use in a body of fluid is disclosed. The electronic system includes a first housing element and a second housing element. The second housing element is hermetically sealed to the first housing element, forming a first chamber there between. An electronic circuit is mounted within the first chamber. The electronic system further includes a second chamber internal to the second housing element. The second chamber is configured for holding a power source for the electronic circuit. Preferably, the power source is a battery pack. Also, the electronic system includes means for temporarily sealing the second chamber. Preferably, means for temporarily sealing the second chamber includes a detachable plug configured for coupling to the second housing element.