From Wikipedia, it appears the only company operating Split-Vis ferries is Jadrolinija

From the local lines' timetable on their website, are operating slow car ferry line 602 (taking 2h20) and fast catamaran line 9602 (taking 1h25-1h35)

do I need to book them in advance or should I just show up?

I do not know for the car ferry, but for the catamaran, their reservation system lists how much seats are left when looking for a crossing.

Looking at answer writing time, for the following week on those catamarans crossings there are always more than 200 seats available, except on Tuesday 25th where there are less than 150 seats

From Wikipedia, it appears the only company operating Split-Vis ferries is Jadrolinija

From the local lines' timetable on their website, are operating slow car ferry line 602 (taking 2h20) and fast catamaran line 9602 (taking 1h25-1h35)

do I need to book them in advance or should I just show up?

I do not know for the car ferry, but for the catamaran, their reservation system lists how many seats are left when looking for a crossing.

Looking at answer writing time, for the following week on those catamarans crossings there are always more than 200 seats available, except on Tuesday 25th where there are fewer than 150 seats