Instead of buying expensive pool LEDs from the store, you can make one in 15 minutes with a few materials. This is guaranteed to work, and is very simple to make.

Step 1: Gather the Materials

You will need a few materials to make this project.
1 small clear plastic bottle with the lid
1 LED
1 battery pack from a old flashlight
Aluminum foil
Glad Press-n-Seal wrap (or opaque Cling- Wrap)
Tape
Styrofoam

Step 2: Clean the Jar

Wash the jar with warm water so the label can be peeled off. Wipe and set aside to dry.

Step 3: Attach the Wiring

Tape the foil to the ends of the battery pack, but be careful NOT to join the ends. Tape the short piece of foil to the '-' end of the battery pack, and the long strip to the '+' end of the pack. Connect the short piece of foil to the short end of the LED and crimp the edges so it stays in place, and then tape it. Take the second piece of foil and tape to the long end of the LED, as seen in the picture. After this, touch the loose ends of the foil. It the LED lights up, go to the next step. If not, check the wiring, and, if needed, put a rubber band around the battery pack, so the contacts touch properly.

Step 4: Install the System

After the wiring has been completed, gently lower the system into the jar. Then bend the edges around the rim of the jar and tape it.

Step 5: Make the Switch

The switch is a simple way to connect the loose ends of the aluminum foil in a stylish manner. To make it, cover the entire lid with the foil. Then, just screw on the lid. The circuit should be completed by passing the current through the foil, resulting in the light turning on. Screw the lid on to turn on the LED, and unscrew it to turn it off.

Step 6: Making the Jar Opaque

To make the jar opaque, use the Press-n-Seal wrap to cover the jar. This will make a warm glow out of the harsh brightness of the light.

Step 7: Making It Floatable

Take the pieces of styrofoam and glue them around the bottle. This will enable it to float properly. When putting it in the pool, be sure that the foil on the lid does not touch the water, because it will short circuit. Add more styrofoam as needed. If needed, add a circle of styrofoam around the jar, as in the second photo.