Not sure where you sell insurance at but I seen ISO evaluations or audits in PA and VA. We took our department from a ISO of 6 down to a 4 in eastern PA. Their are many factors considered into this. Not just distance to the firehouse. Yes, distance to water and the water available affects it. Everything from water supply, staffing, equipment, and training. All of this affects the rating. I encourage you to look into this. I think it will surprise you what you will find. I spent an hour looking into this and only skimmed what all affects the rating. We were also evaluated on the mutual aid companies we used for a house fire assignment. A pond and a dry hydrant is beneficial to you and your neighbors. Especially if you have the pond already. The dry hydrant can be paid for through a grant from DNR.
I sold insurance in NE PA. We may very well be talking apples and oranges here. An insurer is MUCH more interested in the ISO rating of the PROPERTY we are insuring than the ISO rating of the RESPONDING fire dept.
It comes down to a couple of basics. First is if the proposed insured is within 1000 feet of a fire hydrant, and I do NOT mean one in a pond, I mean a CITY PUBLIC WATER fire hydrant. The second basic piece of information we need is, if there is NOT a city hydrant within 1000 feet, is the responding fire dept within 5 miles.
As a rule in town, area's covered by city fire hydrants within 1000 feet, the ISO ratings are 4-6. For out of town but within 5 miles of a responding fire dept the rating is a 7-9, beyond 5 miles is a 10.
So we have 3 basic classes to deal with. In order to jump classes, from say a 9 to a 6, that would involve running city water lines and installing hydrants to within 1000 feet of all homes in that area.
As a rule there is very little difference in insurance rates among a class, for example a 4 or 6, or a 7 or 9.
So again, you may have changed the ISO rating of your fire dept, but that does not mean you changed the ratings for all areas covered by the dept to have significantly effected the insurance rate, unless you jump those classes I mentioned.
Hope this clears up some things.