Before Christmas I thought about experiment with LEDs. I discovered, how
to power leds with electrolyte. Electrolyte is just liquid which conduct eletric current, when 2 electrodes connected to voltage source are sink to electrolyte. I make video, but belowe are steps.

Step 1: Material

For this experiment you will need:

leds, e.g. 5 mm leds
source of higher voltage e.g. I use just battery with step up module, which convert input voltage to 20 V or 2x 9V batteries.
cables
aluminium foil or anything from aluminium to make electrodes e.g. nails
anything which float on water, like peanuts or polystyrene
water and case

Step 2: Making Floating Led

Firstly, you need to prepare leds. I use peanut cover, which is simple and make holes into. Then insert led.

Step 3: Circuit

I use 20 V module for convert battery voltage (which were 3.3 V) to 20 V. Higher voltage is necessary, as water have high resistance. For electrode I use aluminium foil.

Finally, you can insert leds on water. Because led conduct eletrict current on in one way, led glow only in correct direction (longer terminal on led must be closer to electrode connected to + on battery).