A composite sandwich is merely the joining of several different materials to form a structure. In a very real sense a typical surfboard is a sandwich comprised of foam and fiberglass, held together by polyester resin. One of the biggest problems with this type of construction is that it is not very durable. If you have ever owned a surf style wakesurf board manufactured this way you know that it gets heel dents almost after the first ride. Now, a true composite sandwich with multiple density foam would be comprised of a low density EPS foam core, a layer of lightweight fiberglass, a layer of high density foam and then a final layer of heavier weight fiberglass all laminated with epoxy resin. Get the facts about DIY Wakesurf Board see this.
The higher density foam skin does a few things, it prevents most heel dents and dings. This foam is in the area of 5 pound density, which is considered structural, whereas most surf style wakesurf boards use a 2 to 3 pound density foam which tends to dent under pressure, such as when they are ridden. Also, as discussed below, it separates the fiberglass layers.
This composite sandwich also has more layers of fiberglass, which is what makes any surfboard strong. The most common lamination schedule is one layer of fiberglass on the bottom and two on the top. The composite sandwich will have, as a minimum, 2 layers top and bottom. This extra fiberglass gives the composite sandwich superior strength.
Without belaboring the math involved, the stiffness of any composite beam is determined by the distance between the facings. That is to say, in referring to a wakesurf board, the thicker said board is, the stiffer it will be. Also, the stiffness increase is exponential. Increasing by a factor of 1 increases stiffness by a factor of 4. In our wakesurf board the core of the board acts like the web of an I-beam and the fiberglass skin acts as the facings. The composite sandwich improves on this, by placing a high density foam skin between the two layers of fiberglass on the deck. On a normal surfstyle wakesurf board there is no distance between the two layers, and so by increasing the distance the stiffness is increased drastically, as well as improving strength and dent resistance.