Devonport's city centre is small and walkable. The waterfront, Rooke Street, and the Mersey Bluff are all within comfortable walking distance of each other and the ferry terminal.
Beyond the city, a car is essential. Northwest Tasmania — Cradle Mountain, the Tarkine, Stanley, Strahan, and the west coast — is not accessible by public transport in any practical sense. Most visitors arrive on the Spirit of Tasmania with their own vehicle, which is the right approach. Car hire is available at the ferry terminal and at the airport if you've flown in.
Merseylink operates local buses within the Devonport area, but the routes and frequency are limited and unlikely to serve most visitor needs. The Redline coach connects Devonport with Launceston and other towns on the north coast, useful if you are travelling without a car and sticking to the main population centres.