You shouldn't count on it having ferries. Rail Planner/Interrail exist to show trains (and even then, they're sometimes missing but that's another topic).
The ferry is not included but you get some discount. See https://www.interrail.eu/en/plan-your-trip/tips-and-tricks/trains-europe/ferries/stena-line-ferry
It's a great route, esp. considering you save overnight accommodation elsewhere. The process is straightforward. There are jetbridges so no buses required to get on or off.
Some pictures and details here: https://www.seat61.com/trains-and-routes/london-to-amsterdam-by-ferry.htm (excellnt website)
For planning, research separately Edinburgh - Harwich International and then Schiedam/Rotterdam - xxx the following day. Note that the subway from Hoek costs a few €: contactless card, Interrail not valid.