Ben's right: creating a level surface can cost more than you realize. If possible, dig down to match the lowest point. Building up is far more difficult and expensive. I HAD to do it, using retaining wall block, filling it in with QP (Quarry Product, like a dirty form of gravel) In the end it cost over $5000! But that is for a 16x40 rectangular, and I had build up about 30" at the lowest point. Digging down is much cheaper. In my area you can even rent a Bobcat for a day for a few hundred $$$.

The water hose level is the least expensive way to get an accurate level, but it's also the least practical. If you can rent or borrow either a transit or a laser level they can be much more practical to use. When I did my base, none of the workers could run my transit level, so I set up an inexpensive laser level...they could barely manage that.

BDTD had a nice alternative: Find a straight, long 2x4 (like 16 feet). Make one end the center of a circle, say on a bolt on a short piece of 4x8, put a 4' or 6' carpenter's level on it, and you can swing the arc and make it level. Just be sure to measure the height at the center. One person can use this.

I am much more conservative than Ben: I wouldn't allow more than 1" of deviation from level and would aim for 1/2". The more out of level, the more added stress on the structure.

BTW, when I had my original Intex Donut, I had to build a platform. I had enough debris around from tearing out a rustic stone fireplace (now slate and granite) to build a 16x16 platform that rose about 14" at the low point. Not knowing what I was doing, I filled it all with sand when I should have used QP, with 2-3" of sand on top. Still, it worked for the 3 years I used the Intex. I'd cover it with a staked-down blue tarp over the winter.

Carl