The owner should first check to see if his insurance company will even allow commercial use of his pool for this. They might say that they can't or won't. Once he starts to rent the pool, it becomes a commercial operation.
Because if something happens in his pool, it's not only on you. Ask him if he's ready to lose his house.
I can pretty much guarantee the lawsuit will name him as the facility's owner, so he better have commercial coverage for that. If someone slips and stubs a toe that needs a trip to the ER, that person's insurance is going to go after someone for payment.
Then, he would have to see about getting facility insurance for the pool. Then that insurance company will dictate what coverage you need to have.
DAN used to offer free coverage for OW students in training that any instructor could sign up for. However, as I recall, the training had to take place at a place where training was permitted.
I am not aware of insurance that will cover a facility's liability, which you can carry if they don't have coverage that permits commercial use.
They need to have their own insurance that will allow scuba training and will dictate the coverage you need.
If they don't have coverage that allows an insured person to teach SCUBA, they are taking a very big risk.
Often, if an entity has commercial coverage, it will allow them to rent the pool for any reasonable purpose they see fit. But, if they have coverage that specifies commercial operations are not permitted, and they get caught in a suit where that was taking place, the facility will be on the hook for any damages.
Your instructor training should have spent a fair amount of time on this subject. It's part of the risk management side of teaching. If it didn't, you got shorted on content.