I first heard the term "boatable" from a reader. He sent me an e-mail following up on a column I wrote about investing in vacant land. When I called him back and asked what he did, he said he worked for a company based in Idaho that built "boatable" developments.
Generically, when one sees the word boatable defining a residential property, it means the home or development was built on a section of shoreline that can be accessed by boat, but there are regional differences in the interpretation of the word. Ocean-facing properties don't use the term, so if you see the word, it generally means the development is an inland-type property usually on a river or lake.