Jan 19, 2016 - 3:48 PM
Welcome to MySwissAlps!
It works a bit different actually. Boarding a boat or any other mode of transport is not what makes it a travel day.
If you have a pass with consecutive days (the Swiss Travel Pass), every day is a travel day, whether you actually travel or not.
If you have a pass with flexible days (the Swiss Travel Pass Flex or Eurail Select 4 days in 2 months), you write the date of the desired day on your pass. That allows you to travel on everything covered by the pass. You don't get free boat trips on non-travel days.
For details about how each pass works, please see the above links.