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.

The cheapest is to buy a 2-hour ticket. Because of the distance, the km-based fare works out more expensive.

You can also consider the ferry from Newcastle-IJmuiden (DFDS) and Hull-Rotterdam (P&O). No Interrail discount on those, but worth a look.

The Harwich route is probably the nicest ship, and has excellent rail connections on both terminals. But taking the train all the way to Harwich to get a ferry? The quickest route from Edinburgh-Harwich is usually via London - and at that point I would be walking next door to get the Eurostar, rather than trekking across London to get a train to Harwich.

In either case, just check out the ferry times on the individual website, then search for suitable train times to Newcastle, Hull or Harwich International. (In Newcastle or Hull, you’ll need a bus or taxi to the port)

Awesome, thank you all!