Ceredigion council is considering cutting the full-time post in Aberaeron to save about Â£30,000 a year.
More than 600 people have so far signed the online petition.
The council, which also employs harbourmasters in Aberystwyth and New Quay, said it needed to respond to the "tough financial climate" but no firm decisions had yet been made.
The petition argues people's safety would be put at risk, and the number of people who use the harbour - and spend money locally - would also go down.
The proposal is due to be discussed at a meeting of the thriving communities scrutiny committee on 28 September.
A council spokesman said: "The council is in a period of transformation, with the aim of responding as positively as we can in the current tough financial climate.
"This will have an impact on a wide range of services provided by the authority - in a number of different ways.
"At this stage, no firm proposals or decisions have been made about the harbourmaster service in general, or the arrangements for how it is provided in Aberaeron."
Hundreds of residents and visitors to a mid Wales town have signed a petition to save its harbourmaster.