It gives discount-and thats it. The same discount the ferry-comp will probably also easily give to other categories.

Those maps say nothing (alas) I´ve checked a few for other areas and they were even unable to show the raillines.

Also be aware that there are very, very few trains at all in EEstti, that there is 1 awfully bad connection to Latvia and in general even though a railpass is accepted, its better -for you and your purse-to simply pay any trips. Or even better to bus it onward!

The ferry gives a discount indeed and that also means you don't need a travel day. It depends on the ferry company if the discount is also calculated on the accommodation (for an overnight ferry) and also if regular avance fares could be cheaper. So check and compare on the ferry company's website.