Islands can "float" as well. Smaller islands used primarily as an aid in cooking or baking can be fitted with wheels and moved around the kitchen as needed to aid in cooking. Whether you consider a floating island will probably depend on what you have installed on the floor now, or plan to install (a soft wood will be nicked and scratched).
For example, we have seen islands of concrete when the rest of the kitchen countertop is made of granite or another material. We have seen a black island used as a nice "dividing wall" to keep the kids away from a white kitchen.
Kitchen islands are mainly of the wooden or the stainless steel types. They can be used in the kitchen or they can be of the outdoor type where they can be placed in the backyard or next to the swimming pool for preparation of outdoor meals. Kitchen isles can be made of blocks where each block has a particular function or they can be one big unit that incorporates all the requirements into the structure. One can build simple wooden kitchen islands that have drop leaves to save space.
Another problem you might face if you buy your kitchen island as a separate unit, and not as part of the overall kitchen, is matching it to your existing cabinets. This might not be as easy as your think, but if you purchase a solid color, such as black, white or red, then you should get away with it.