Definitely speak with a specialist equine insurance brokerage. I use Broadstone and would recommend them.
Your homeowners will not cover your boarding operation; you will need liability insurance for the boarding business. Liability protects you if someone sues you (if, say, a boarded horses gets loose, gets on the road and causes an accident, and the car’s driver sues you).
Care, Custody, and Control insurance covers you if the boarder sues you because of something negligent you’ve done. It makes sense if you are boarding horses that are valuable. I board retired horses, and though some people consider it heresy, I do not have Care Custody and Control. This is because a) I would have to do something to injure a horse b) the owner would have to decide to sue and c) even if a) and b) happened, the horses have NO value, so what would the owner be awarded? (And for those who say that the insurance would cover legal fees in the event of even a frivolous suit, that is true, but it is also true that the boarder would have to incur substantial legal fees in order to institute a suit)