This is basically what the name says, it's a water heater.

Tests: For the tests I used the water heater vs a regular bottle, each had one cup of water in it
The starting temperature of the water was 20C(68F)
The outside temperature was 10.5C
After half an hour the regular bottle was at 18C(64.4F), and the heater was at 26C(78.8F)
I would have done a longer test but the sun went down.
*There was a large amount of moisture on the walls of the heater wich reduces the amount of sunlight that can heat it. I poured some salt into it which will hopefully dry it out.

Step 1: Supplies

-A glass bottle
-A can of spraypaint
-Two clear 2 liter bottles
-Caulk
-Electrical tape
-A rubber hose and a small plastic tube(or a slightly smaller rubber hose that fits properly into the glass and plastic bottle)
-A vacuum pump
-Tinfoil
-A piece of plastic or cardboard
-Styrofoam

Step 2: Preparing Your Bottles

Cut the bottoms off both of the plastic bottles, make this cut as smooth as possible.
Take one of the pieces from the bottom and either cut or drill a hole in the bottom large enough to it over the mouth of your glass bottle. If you drill this, make sure you get it right the first time.
Put your bottle on a stick and spraypaint it black. Now wait for it to dry...
Put the bottom of your plastic bottle over the mouth of the glass bottle.

Step 3: The Rubber Hose

The purpose of the rubber hose is to connect the glass bottle to the plastic bottle(while the glass bottle is in the plastic bottle)
The hose I used fit perfectly in the plastic bottle, but had the same diameter as the opening of the glass bottle.
The small plastic tube takes care of this problem, though. I made it by cutting the end off a small test tube.
Cut off a piece of the rubber hose.
To attach the hose put caulk inside the it and push the plastic tube in(it shouldn't be a tight fit, because that would crack the tube, but it definitely shouldn't be loose)
Then put caulk on the rim of the rubber hose around the plastic tube and push it onto the glass bottle. Wipe off the excess and let it dry.
Wrap electrical tape around it.

Step 4: Bottle Support

Take your piece of plastic/cardboard and cut it to the size of the plastic bottle. Place tinfoil on one side of this, and spraypaint the other side black. Glue the bottom of your glass bottle to the center of the foiled side. This will hold the bottle in the center of the plastic bottles.

Step 5: Finishing With the Glass Bottle

To seal your rubber hose to your plastic bottle, put caulk inside the mouth of the bottle and push it over the rubber hose and glass bottle. Push until there's about 4.5in. of rubber hose in the bottle, or until it gets hard to push. Wipe off any excess caulk and cut any excess tubing, you must be able to screw the cap on.
Allow it to dry.

Step 6: Close It Up

To close up your heater, place your second plastic bottle over the glass bottle and force the edge under the first plastic bottle. You will see an indentation in your second bottle, remove the bottle and cut a V where the indentation was. Now put caulk around the edge of the second bottle and put it back in place.

Step 7: Reflector

For a reflector I would recommend something like mylar tape, but I don't have any so I used tinfoil instead.
Cover half of the heater in foil.

Step 8: The Lid

The largest amount of heat will be conducted from the water by the lid, so it should be insulated. Take a piece of styrofoam, cut a hole in it, and glue the lid to it.

Step 9: Use

The reason why you vacuum out the air is to improve the insulation. So now you should vacuum it. After sealing any leaks and vacuuming it you can put your water in it. Place it any way you want as long as the bottle is facing the sun, but it may work better if the opening of the glass bottle is facing up.