Hello 2021bryant
Do you know the Swiss Travel Pass validity map? There, you can see where it is valid (see red / yellow / blue / black solid or dashed lines; on dotted routes, it grants a discount; on grey routes invalid).
https://www.sbb.ch/en/tickets-offers/travelcards/area-validity-map.html?layers=ch.sbb.geltungsbereiche.mvp-sts
You can use the Swiss Travel Pass between Schaffhausen and Kreuzlingen on the Rhine river (if the boats operate). I suggest that you should do a lake cruise on one of the steam paddle boats on Lake Lucerne, there is no better boat experience!
Please let us know if you have got mire questions.
Kind regards,
Roland