Step 1: Go to the Fishing Section
Step 2: Eyedropper
Get yourself an eyedropper, this one was glass I'm sure a plastic one would work, you just may have to add a weight or something to the bottom
Step 3: Wiggle Them Together
Step 4: Suck Up Some Water
Squeeze the bulb of the eyedropper and get about enough water to fill just the bulb, this may take some trial and error, to get enough water that his head just comes above the surface
Step 5: Neutrally Buoyant
This is what it should look like to be neutrally buoyant, prepare for awesome. (the term neutrally buoyant brought to you by my dive instructor Neil, and the letter E, for Ed, Ed owns the dive shop, Ed and Neil are good peeps.
**OH! I originally did this with a 2L pop bottle (yes, pop, I'm Canadian) but it was really hard to make him dive, so I swapped it to a 500ml bottle and that fixed the problem
So obviously the 'science' in this is buoyancy and density, if it really boggles your mind you can read more about traditional cartesian divers here: http://en.wikipedia.org/wiki/Cartesian_diver yeah yeah it's a wiki link but it seems to have all the right info.
Go, be with squid, be with bliss
Here is the little guy in action!