A £6 million new vessel intended to service the Strangford ferry route is still not in service as problems with the ramps mean cars would be unable to drive off it at high tide.
Hundreds of journeys are made using the 10-minute link between Portaferry and Strangford every day and there is no plan B if the only serviceable boat malfunctions, a local representative said.
Figures from Stormont show that current vessel has broken down at least 35 times this year.
Alliance Party Assembly Member for Strangford Kellie Armstrong added: “I hope it keeps working. If it does not, the minister (Chris Hazzard) is in trouble.”
Official figures obtained by the AM revealed dozens of stoppages on the lifeline route used by school children and hospital patients.
Ms Armstrong said there were 35 breakdowns this year to November, 43 last year, 51 in 2014 and 59 in 2013.
She added: “The boat is not getting a break until 10.30 at night. The Portaferry II boat is back and forward and back and forward and the plan B was supposed to be the new boat.
“There is no plan B.”
Stormont Executive paid £5.7 million for the bespoke ferry built by Birkenhead-based Cammell Laird.
The firm describes itself as one of the most famous names in British industry and dates back to 1826.
A spokesperson for the shipyard told the BBC: “All handover requests were made. We delivered what we were asked to deliver. The ramp meets the specifications we were given.”
The new vessel, called the MV Strangford 2, was originally scheduled to begin operating last summer but will remain docked until the repair work is carried out.
It can take around an hour and a half to travel from Strangford to Portaferry by road via Newtownards. The ferry crosses the mouth of Strangford Lough to the tip of the Ards Penninsula in minutes.
Allison Murphy, a director of the Portaferry and Strangford Trust community organisation, expressed concern.
She said school children from Portaferry and those attending medical appointments relied on the crossing every day to travel to Downpatrick.
“It is vital that they sort this out very quickly. They have known about this for at least three months.
“If the current boat breaks down we have nothing to fall back on.”
A spokesperson for the Department for Infrastructure said: “Work to complete these modifications (to the ramps) is currently underway and discussions are ongoing with the shipbuilder to resolve this issue under the terms of the contract.”
The department said it regretted the delay and expected the boat to enter service early in the New Year, adding: “Until then, the current ferry remains fully operational. There is also a backup vessel available if required.”