You should divide by the water's temperature in K. This answer is pretty close to Cort Ammon's (Cort Ammon seems to be dividing by how much you measured your water to be above absolute zero, rather than how much it actually is). My reasoning:
How can you calculate absolute zero to be 15K? 0K is defined as absolute zero. The only way your calculations makes sense is you're trying to calculate how far above absolute zero you are. For instance, suppose your water is at 2°C, and you're trying to use its properties to find AZ. You say that you found that AZ is at 15K. Well, first I'm going to reinterpret that as "I found that AZ is at -258°C", because, again, "I calculated AZ to be at 0K" is nonsensical. Next, I'll interpret "I found that AZ is at -258°C" as "I calculated AZ to be 260 degrees below 2°C". Note that although these last two statements are logically equivalent, the percent error calculations they suggest are quite different. In comments to Cort Ammon, you ask why the temperature of your water isn't as arbitrary as the Celsuis scale. The answer is that this is how far you're trying to extrapolate out. Hopefully, if you had had water at -100°C (okay, that wouldn't be water anymore, but you get the idea), your error would be smaller. It is reasonable to compare how far off your extrapolation was to how far you extrapolated. You were trying to extrapolate to something that was 275 degrees away, and you were 15/275 = 5.45% of the way off.
So in this example of water at 2°C, presumably at some point in your calculations, you found that you were 260 degrees above AZ, and then took the "known" number that your water is at 275K, and took the difference to find AZ = 15K. Or you did some equivalent calculation. You are looking at the final number of 15K and asking what percent off it is, but the more meaningful number is what percent off your intermediate calculation of 260 is.
Asking "How do I calculate percent error for absolute zero?" is a bit like asking "I tried to calculate the position of the center of the earth in a coordinate system in which the center of the earth is the origin. How do I calculate percent error?" If you calculated that the center of the earth is 6,356 km away, rather than 6,371 km, then your percent error is 15 km/6,371 km. Saying "I calculated that the center of the earth is at r = 15 km, but it's actually at r = 0 km. So the percent error is 15/0." is fallacious. Yes, your distance from the center of the earth is in some sense "arbitrary", but it's still the valid denominator, because that's what determining how far out you have to measure.
Any time you have a number that represents how far away you are from a point, whether it's Kelvin measuring distance from absolute zero, or r measuring distance from the center of the earth, measuring percent error with respect to that number will always get you infinity. So instead of measuring distance from that reference point, you should measure distance from the point you started from.