Troutgal,
I will respond to your question in regards to "bentonite thickness". Actually, the question you should be asking is "how many pounds of bentonite are required". You will need 3.5 to 4 pounds per square foot of surface area to be treated. Water depth is not an issue, so don't try to make this rocket science. A good quality bentonite meant for pond or lagoon sealing will do the job, and expect to pay from $7-$9 per 50lb bag. You do not need to pay $20-$30 per bag as I read on a previous post. I would not recommend buying bentonite from the local farm mill as this bentonite is used for something other than pond sealing. No sense paying $2 per bag less only to find out the pond did not seal. Hope this information helps.