Nantucket has some of the strictest zoning and architectural restrictions in the US and probably the world. So, when an aspect of a property is grandfathered, it usually makes that property more valuable than the same property without that specific aspect grandfathered. For instance, properties are allowed a certain ground coverage percentage per their zoning. If a property pre-dates zoning and is over the current ground coverage percentage, it will have the ground cover overage grandathered and therefore be more valuable than the same property that is required to conform to the current ground coverage ratio.
There’s an Article on the Warrant of Annual Town Meeting that, if passed in the current state, would heavily restrict the installation of pools in most zoning districts and outright ban them in one district. I doubt it will pass because it is so far reaching, but if it does, anyone currently with a pool will be grandfathered and see a windfall of value. If it doesn’t pass, it will probably make owners who were on the cusp of installing a pool, install one because of fear that some form of this article will get passed in the future. So, in that regards, at the least in the short term, it will have the opposite effect of having less pools.