Auckland being a sprawling city doesn't have the best train/public transport system and network and hence most people drive to work; resulting in some pretty long traffic jams mostly on the motorways. If you are interested in taking the train to work and want to live in an affordable rental suburb, you might consider Western Auckland ie Waitakere City. Massey, West Harbour, Hobsonville are possible considerations and you can drive to the train station and commute. This rather than you live south of Auckland city, as a possible consideration.
However I find the ferry the most pleasant way to commute mostly because there is no road jams and certainly it is a more delightful way to travel and see the beauty of the harbour in the morning. If you can get an affordable rental in the Devonport area, the ferry is the way to go. Beware though the suburb is keenly sought after mainly because people really like living there. You can even bring your bike on the ferry, something you can't do on the bus.
For a more affordable area, consider the alternative Birkenhead ferry which is also a pleasant journey though parking can be the problem. Birkdale, Beach Haven used to be low socio-economic suburbs but in the last 2 years or so, have risen up quickly due to more middle-class, younger families moving into the area, being pushed out , daunted by the higher prices found in almost all of the North Shore. So finding a more reasonable rental is possible.
There are also ferries from Manukau and even as far away as Gulf Harbour. Some people consider the alternative of living on Waiheke Island (real estate there is cheaper) and commuting to work by the ferry. However such journeys cost more and are far longer.