Using Photoshop to measure the incident and refracted angles. Based on a reference I found online, the measured index of refraction compared to the expected values were off by less than 1%. Not bad for a quick and dirty setup. Check out how close I managed to get the lasers aligned, too.
On a side note several materials fluoresce in the blue laser light. So I put a few drops of fluorescent dye into the tank and waited for something interesting to happen.
Oh, and by the way - some of this refraction stuff is why you can get a "green flash" at sunset. You can find some more on the hyperphysics website at Georgia State and a detailed review of the phenomenon by Andrew Young.
When my wife and I were in Hawaii, I managed to see the green flash on three different evenings over the 10 days we were there - not a bad average.
And now the flash: