They're called boaterhomes. I use the plural form because, surprisingly, this isn't a one-on-a-kind vehicle. There are many of them. Like the saucer separation procedure in Star Trek: The Next Generation, a boaterhome drives to the shore, then detaches a fully functional cruising boat.
This particular model, the AutoBlog tells us, is a 1987 Watercraft Sports King that rides a 1986 Ford E-class van. All it needs is helicopter wings to offer a full menu of transportation options.
-via Geekologie