I expect BoatUS will be fine. They don't have a hard sell. As you are most worried about liability (quite appropriate), you'll don't have to worry as much about the insurance on the boat itself. And an old Ranger 26 - while a nice boat - is probably not going to impoverish you if it sank. So you can reduce the premium quite a bit by taking a high deductible on the hull insurance - that won't effect the liability part. Oh - and a very important reason to have insurance is that they pay to recover the wreck of your sunk boat and any pollution fines from leaking gasoline - this is often more than the value of the boat.
Also, do you have an umbrella liability policy on your house and cars? If so, I would call your insurance company and see if they are competitive on the boat insurance. Many companies don't advertise boat insurance but are happy to do it. And my home insurance company (Amica) will add small boats onto the homeowners policy with no extra charge. I doubt 26ft is small enough - but worth checking.
And if you do have an umbrella policy, talk to your insurance company about adding the boat to the umbrella policy even if it's insured with another company. They usually say OK if you send them a copy of the boat insurance policy and the amount of liability on it is high enough (usually something like $300,000). They'll then cover liability over the $300,000 as part of the umbrella for no extra charge.
And not to be scary, but bad things can happen on a boat - like a child drowning - so you want plenty of liability coverage.