Registration will require the EEC conformity certificate which you should be able to obtain from the boat dealer or the manufacturer.
Not 100% clear yet, waiting for feedback from Zurichsee (and the rules for your lake may be different), but it looks like you wouldn't have to register if you stay within the regulated area which extends a hundred meters from the edge or shore. As it's a sailing boat I guess you want to cross the lake and go where you please in which case registration is necessary.
Contact the local Seepolizei again - best to go in and speak to a face for better service

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