I got curious to find out whether there is anywhere else in the world that is in a similar situation as Waiheke Island, i.e. a good-sized commuter population that is dependent on a ferry service to get to its main workplace. I'm surprised it's rather difficult to find and to compare prices as many different options exist and passes cover more or fewer other transport modes too (perhaps you could make me some suggestions).
In New Zealand, there is the Wellington ferry to Eastbourne, which costs $245 a month. Of course, Eastbourne commuters have plenty of other transport options.
Overseas, there is the Seattle to Vashon Island route. A monthly pass (valid for 31 return rides in a calendar month) costs US$118 (NZ$166). Comparison with Waiheke is difficult: there is no weekend service to Vashon and only three sailings a day during the week. The pass also doesn't cover any other transport mode. The monthly pass to Fauntleroy (West Seattle), which has many more sailings, costs US$55 (NZ$78).
The San Diego to Coronado ferry is free to commuters.
Many ferry services are to islands which also have road access (and thus an alternative for commuters), i.e. the Staten Island Ferry in New York, which is free of charge. The Sydney ferry system is another. Yearly combined ferry/bus passes are between A$1,280 (NZ$1,562) and A$2,200 (NZ$2,684) depending on the zones travelled. Inner harbour ferries in Boston cost commuters US$198 (NZ$279) a month (Commuter Boat Pass also good for unlimited travel on Local Bus, Subway, Express Bus, Inner Harbor Ferry, and Zones 1-4 on Commuter Rail). None of the destinations are islands.
The Alameda-Oakland ferry in San Francisco charges US$170 (NZ$240) for 20 return trips within a calendar month
The Hong Kong Star ferry between Central and Tsim Sha Tsui costs HK$110 (NZ$20) a month. Of course there many other transport options in Hong Kong and the public transport has the wonderfully integrated Octopus Card system.
In Vancouver all-mode monthly passes (including SeaBus) cost between C$73 (NZ$99) and C$136 (NZ$184) depending on the number of zones.
In Scandinavia, Malmö has now become a suburbs of Copenhagen, thanks to the Øresund bridge/tunnel linking the two. Ferries are not really the preferred commuter option there.
In Maine, Peaks Island Frequent Rider costs US$87 (NZ$123.50). Cliff Island costs US$127.50 (NZ$181).
Isle of Wight (Ryde to Portsmouth) fast ferry costs £150.80 (NZ$392)
So, in summary, the Auckland to Waiheke commuter ferry service is one of the most expensive stretch of water in the world. Regards the Isle of Wight fare, one should consider UK wages are way higher than in NZ, and so I would still posit that Waiheke actually is the most expensive fare in the world.