I just went through the process of getting my 15' Avon certified to use the State Parks Department hoist at Gaviota beach north of Santa Barbara, Calif.
From what I can gather, they have the most stringent requirements of any hoist in the state (perhaps the country).
It was a real hassle to set up the boat according to their specs and take their training class, but in the process I've learned more about hoists than I ever wanted to know.
Here's a link to their operator's manual, which might be helpful:
http://www.parks.ca.gov/pages/606/files/Revised_20100423.pdf
The U bolts on the transom had to be backed with 3" square 1/8" thick stainless plates. On my boat the bolts are about 6" below the top of the transom.
For soft-hulled boats, the hoist requires that the front sling has to go completely under the hull (known as a basket sling) and that there are D rings on the outside of the tubes to prevent the sling from moving around.
I had someone with a forklift raise the boat with a temporary sling so I could find the center of gravity and figure out the lengths for the sling legs. I gave those measurements to a rigging company and they made me a sling accordingly.
You can see some pictures of my boat being raised here:
http://s47.photobucket.com/albums/f164/jondavies/Boat%20Lift/
I would talk to the operator of the hoist you are planning to use and see what they require/recommend. However, if you follow the guidelines for the Gaviota Hoist, I'm sure you will be covered for any eventuality.
Let me know if you have any questions.