We are taking a family vacation starting Saturday and will take the ferry from Dubrovnik to Split on Friday August 16, 2019—four adults and one nine-year-old. Where do we buy tickets, and do we need to get them in advance?
Take a look at the schedules on www.krilo.hr. There is an English language option (British flag) in the upper right of the home page. Choose the "Split-Dubrovnik" sailing schedule and the page will also show you the Dubrovnik-Split option. With Krilo, there are departures at 4:00 and 4:30 pm. You can purchase tickets on the website. The boat makes four other stops before Dubrovnik. If you want to go earlier in the day, the bus is a fine option. I took the bus in June, 2018, and it was comfortable, and fast; a morning departure bus fit my schedule better than did a late afternoon catamaran ferry.
The catamarans arrive in the evening. Taking the bus gives you more flexibility and you can get to your accommodation in daylight. BusCroatia.com for timetables and tickets.
There's also a Jadrolinija catamaran departing at 7am. The Jadrolinija ticket office is out at the port, but you may be able to buy tickets at a travel agency or thru their website. I always buy catamaran tickets upon arrival in a city, but I haven't traveled in busy August. In case the early morning or late afternoon times don't work for you, the buses from Dubrovnik to Split are fairly frequent and quite comfortable. The long-distance bus station is at the port, an easy local bus ride from the Old Town.
Can’t say enough good things about Krilo ferry. https://www.krilo.hr/en/. Everything was on time. We bought our tickets in advance so no issues at the port. They are like sitting in airplane seats and you hardly notice them coming and going into ports. You ride in an enclosed cabin so all climate controlled. I was expecting to go out on deck and enjoy views but you can’t do that due to speed of the boats.