The beauty of using the TMP300B in a project like this is its wide operating voltage range. I designed the PCB to take a standard 5mm and 0.1" spaced power supply, so you can power it with just about anything ranging from solar cells to supercapacitors. Just make sure that your container is water tight and weighted down so it sinks to the bottom of your water container. Alternatively you can pot the PCB in food safe silicon and power it externally.