A floating drive-on watercraft dock comprises a one-piece molded body defining a watercraft receiving area. The watercraft receiving area includes roller assemblies on the bottom of the watercraft receiving area and glide assemblies on the sides of the watercraft receiving area. The roller and glide assemblies can be easily removed and replaced for servicing of the watercraft dock. An extension unit is provided which can be connected to the watercraft dock body. The extension unit includes an extension body and a tongue extending from the extension body. The tongue is sized and shaped to have a bottom surface complementary to the entrance to the watercraft receiving area of the watercraft dock body. The extension unit also includes a watercraft receiving area, which, when the extension unit is connected to the watercraft dock, increases the length of the watercraft receiving area.