Hi Bill,
I have dabbled with the garden hose long ago to see if the idea would work and it did. Wound up with 1200 feet of 1/2 inch black poly pipe configured in 12 parallel 100 foot runs. Made a couple 1x12 manifolds from PVC pipe and adapters. The pipe is layed out on black rolled roofing material in my back field. The configuration is roughly 8x50 feet. The whole assembly is located about 80 feet from the pool and fed with 1.25' buried pipe. Works like a champ. I used tarpaper initially but it soon deteriorated so went with rolled roofing material. As for flow, you actually want as much as you can get, theoretically. You get maximum heat transfer at a maximum temperature differential so the cooler the pipe is the more heat it absorbs. From a practical standpoint you can cut the flow but you don't want a big temperature rise. I forget what my numbers are but I know my flow is less than 10 GPM and I remember the rise was around 4 deg F. The 1/2 inch black poly comes in several pressure grades so use the cheapest you can find. Cheaper means thinner walls which is better too.
Hope this helps.
Al