Rick Steves states that there is no need to reserve seats for the boat or train. However, when I called Rail Europe to check on tickets for Glacier Express, etc I was told that tickets were mandatory for William Tell. So I have some on hold but they cost me $84.00 for two!
The same was true for Bernina Express. Those seat reservations cost me $64.00. So is it necessary or not to get these seat reservations for the William Tell and Bernina?
From our experience on the Glacier Express, even if you walk up to the ticket window and buy a ticket (which we did 1 day in advance), you have to pay a reservation fee. For us last year, there was a 15CHF fee or a 30CHF fee. The 30CHF fee got you onto the special observation car with audio (which came on twice during the run for 30 seconds. It wasn't worth it). The reservation fee is nothing more than a guaranteed seat reservation. As I mentioned above, you can walk up to the Swiss or Italian ticket window and buy your tickets for these special express runs a day or two before your travel on them.
Also note that local trains without reservation fees run on these same tracks for less money. You might have to change trains a couple of times but it can be done. For us, we had a one-way ticket on the GE from Brig to Disentis that went over the Oberlap pass. Very scenic. At Disentis, we returned to Brig on the local regional train.
I'm having helpful emails with the RhB, which runs some of the scenic lines including Bernina Express (which itself is NOT a single line!)
http://www.rhb.ch/index.php?id=891?&L=4
My understanding is that you need reservations, though not clear to me if you can probably get those at the last minute. I think the scenic glass viewing cars book up weeks before, but regular seats are often available last minute, though not sure yet.