Please check this page: https://www.interrail.eu/en/plan-your-trip/trains-europe/ferries.

Ciao. I do not really understand what you mean. You CAN use a ferry-there are several from avrious ports, to HR but a railpass is NOT valid but may give a small discount-most often you also get the descuenta as ju/senior. ONLY Split has still trains going inland!  If you mainly want to explore the coast/costa along Hrvatska, then a pass is of no value.