"Waterfront" could mean the property is on a beach at the edge of a sim, or, it could be land covered by water. In order to find out whether this is land that truly has water access, you need to look at the property around it.
How to de-render water? Control plus Alt plus D brings up the Advanced menu on the Second Life viewer. (Information from a post by Lindal Kidd). Click "Rendering types" then click "Water" and water will disappear.
|How to find the 'de-render' feature|
|Same scene, with water de-rendered.|
|Worldmap - property in question at lower left|
Out of curiosity, I checked out the 512 parcel shown in yellow near the upper right of the map. Its north property line is right on the open water, so you can be sure you have water access; however, the asking price per meter is over 30 times that of the property on the lower left.
While many private estate owners have set established waterways with restrictions so that their residents can sail freely, when you buy "waterfront" land in the Mainland, the only way you can be certain that you will continue to have water access is if your property is right on the border of Linden Maintenance land. Fortunately, most people do not block access past their water properties.