Hythe Ferry runs between Hythe and Southampton, serviced by a train which runs along a 640m (2,000ft) pier.
The presenter lent his support to a community group's aim to take over the management of the pier and train.
Its current operator said numbers using the ferry had been falling.
Earlier this year, Hythe Ferry Ltd warned staff about possible redundancies, having suffered a "year-on-year decline" in passenger numbers and faced with higher operating costs.
More than 9,000 people have signed a petition calling for the service to be saved.
Peter King, of the Hythe Hythe Pier Train and Ferry Action Group, said members wanted to create a "viable modern ferry" by a charitable trust taking over the management of the Victorian pier and "relieving" the ferry operators of the costs of maintaining it.
He said a redevelopment project could cost £2-3m but a trust would be able to access other sources of finance, including lottery grants.
Mr Snow said: "This train is the oldest running pier train anywhere in the world, so I'm campaigning to keep it open. We need to do everything we can to keep this extraordinary piece of our past running."

The three-month tests of 137 island homes, 73 in Guernsey and 64 in Jersey, measured the level of gas in the walls.
Radon occurs naturally in areas with a large amount of granite and 99 of the homes tested were below target levels.
Fourteen island homes were above the target level and owners have been told to ensure there is good ventilation.
The survey was carried by the islands' authorities with the UK Health Protection Agency. The 137 properties were selected to cover a range of geological conditions and population areas.
The aim was to find out more about radon and its distribution.
Val Cameron, the Channel Islands strategic lead for environmental health, said: "The recent survey updates and confirms the information from previous surveys; this is that radon is associated with the granite geology of the island, and not the construction material of an individual building."
International research has found exposure to radon gas can increase the chances of contracting lung cancer for people who smoke.
Dr Susan Turnbull, medical officer of health for Jersey, said: "To help put the main risk factors for the commonest form of lung cancer into perspective, for every 100 cases of lung cancer around 95 will have been caused by smoking alone, about four will be due to the combined effects of smoking and radon exposure.
"Only one will be due to radon exposure alone. So it is a real risk, albeit a low one. The most sensible thing anyone can do to reduce their risk of getting lung cancer is to stop smoking."
Mrs Cameron said: "Of the homes surveyed, some were identified as being above the action level. Advice has been given to those householders about measures to remedy the problem."
Previously radon surveys were carried out in Jersey in 1987, 1992 and 1997, and in Guernsey in 1985. In 2012, tests were also carried out in Herm, Alderney and Sark.
In 1984 the Royal Commission on Environmental Pollution and the International Commission on Radiological Protection reported on the need for a reduction of radon exposures in homes.

The 21-year-old was also suspended for four months backdated to 23 May although three of those months have been suspended until 1 July 2016.
Townsend had already voluntarily withdrawn from the England squad for the European Under-21 Championship.
It is believed he was not accused of betting on games he had involvement in.
However, the FA has strict betting rules that forbid gambling on a wide spectrum of fixtures laid out in its regulations.
The length of the up-front ban, coupled with the fact that he withdrew from the England Under-21 squad, mean he will not miss a game.
The midfielder, who was on loan at QPR for the second half of the season,  is a product of Tottenham's academy and signed as a trainee in 2008.
He has had loan spells at Yeovil, Leyton Orient, MK Dons, Ipswich, Watford, Millwall, Leeds, Birmingham and, most recently, with former Spurs manager Harry Redknapp at QPR.
Townsend made his Premier League debut for Spurs in September 2012 before joining Rangers in January, where he made 12 Premier League appearances, scoring twice.
A Hampshire pier and ferry service facing an uncertain future is a "national treasure" which should be saved, television historian Dan Snow has said.
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The amount of radon gas found in the Channel Islands is associated with the geology of the island and not construction materials, a survey finds.
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Tottenham midfielder Andros Townsend has been fined £18,000 by the Football Association for breaching betting regulations.