Does a floating platform/floating island need to be registered?

Yes you do need to register (or more accurately doent) this "island'. In fact, if your not a navel architect, you'll soon learn that there are a lot of regulations regarding it's construction. I personally love you "Green" idea of plastic bottles, but I fear that the USCG would not permit you to use this as planned. I'm unsure where to point you, because as you say the regulation are hard to understand if you haven't spent years building boats (in this case your Island is considered a barge). Try the USCG again......