This is an easy way to power an highly brightness LED for 10 hours, some parts are recycled or was buy from ebay for 1$ or less …

– Solar panel 5v – 0.5A (ebay)
– 5V Mini USB 1A TP4056 Lithium Battery Charging Board (ebay)
– LM2596 DCDC Converter (ebay or dx.com)
– A Li-On 3.7V batteries (recycled from old phone)
– A photo-resistor, 10k pot meter, 1K resistor pull down, highly brightness 10mm LED (white warm), cables, and small box…

Connect solar panel to TP4056 charger input, connect batteries to output.
Connect output of charger to DCDC converter input, connect the LED to output.

Put the LM2596 Pin 5 to GND with a 1k resistor, connect the LDR and pot-meter resistor in serial, then connect that between LM2596 Pin 5 and IN+. When the ambient light decrease, the pull down resistor turn the DCDC converter to ON. You can adjust sensitivity with the pot-meter. I use a highly brightness 10mm LED, under 3.3v @ 90ma.

Water box.