You can easily register the
boat in the US in CA. The boat does not be in CA to be registered there. CA may want to collect
taxes on the boat. Check with your local
regulations.
However, if the boat stays in Greece permanently, even if you only use it occasionally you will have to deal with Greek
regulations. If you are
buying the boat in Greece I am assuming the EU VAT is already paid and the boat is certified to meet EU
safety requirements.