For the record, a ferry sails every Sunday at 7am for Gorgora (Birr278, 1½ days), on the northern shore of Lake Tana, from the Marine Authority Boat & Ferry Launch, but there are far better ways to get there and to see the lake. It stops for loading at Dek Island (with enough time to wander around) and overnights in Kunzula, where there’s food and a couple of grotty hotels. It’s far from luxe, with a toilet and limited seating in what they call ‘1st class’. Snacks and drinks are sometimes available, but it's best to bring your own. Buy tickets from 2pm to 4pm the day before at the Marine Authority office.
Minibuses travel at night (which makes them dangerous) and can do the trip in as little as eight hours. Your hotel can reserve a seat for you, or ask around for an agent at the north gate of the bus station to avoid the hotel’s commission.
There are two more comfortable options:
Two buses travel to Gonder (Birr75, three hours) at 6am and minibuses (Birr90) go about hourly. For a steep fee, most hotels will call and have the minibus pick you up so you can avoid the chaotic bus station. Some travellers prefer to band together and hire a private minibus to Gonder, which can cost as little as Birr1200.
There’s nothing direct to Lalibela, but you can take one of about five morning minibuses (Birr150, three hours) or the 6am bus (Birr145, 3½ hours) to Woldia and get off at Gashena to catch a connection there.