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."
A Hampshire pier and ferry service facing an uncertain future is a "national treasure" which should be saved, television historian Dan Snow has said.