The way I understand it, one home per 5 acres doesn't mean 5-acre home lots. It means you can subdivide a 5-acre lot into a regular-sized home lot (like 1/4 acre or 1/2 acre or whatever) and leave the rest untouched. So if you own 50 acres, you can get 10 home lots out of it. If the lots are 1/2 acre each, that means 5 acres for home lots and 45 acres left as farmland or forest.
So the density of the zoning really doesn't have a lot to do with the size of the lots. Now, decreasing the number of home lots available in the rural parts of St. Mary's may drive up home prices, if the county has lots of demand for all types of housing.
About buying retirement property on the water--I think the local zoning density doesn't have that much to do with the pricing. What does drive up the cost of waterfront homes? There are plenty of state and federal restrictions on waterfront development here, to protect the Chesapeake Bay. Some of it is well thought out and some of it isn't.
About 40 years ago, many people from the D.C. area began buying retirement properties along the St. Mary's shoreline. These lots didn't cost a lot back then. When the waterfront restrictions were put into place in the late 1980s, many of these retirees found that their tiny homes were worth hundreds of thousands of dollars, with the accompanying property taxes. Part of this was the overall trend toward gentrification of the shoreline areas, like on St. George's Island.
