They're still very slow, especially compared to aircraft. Most cruise ships have in-service speed of just over 20 knots, so about 40-45km/h. Ferries/Ro-Ro ships are somewhat faster, with average speeds of around 25 knots, maybe up to 30 or so. When I worked for a ferry company, we had a few high-speed Ro-Pax ferries in the fleet; those were operating at speeds of around 40 knots (roughly 75 km/h). There are a few newer ones now that can reach 50 knots and more. So at best, you'd be travelling at max 100km/h. Your average commercial flight goes around 800-900 km/h. So for a 3 hour flight, around 2500 km, you'd need at least 25 hours on a high-speed ferry, and that's if you are on the fastest ones. Around 35-40 hours, more realistically.