A small patch at the eastern end of Watermans Park, in High Street, Brentford, would be used to provide 14 car parking spaces, along with refuse storage facilities.
Councillors agreed to appropriate the public land, subject to public consultation, at a cabinet meeting on Wednesday (May 20).
The new 26-berth marina was approved in principle by the Labour cabinet in September last year, though a planning application has yet to be submitted.
Its construction would mean the eviction of existing houseboat dwellers, who the council claims are there illegally.
getwestlondon understands the council has already issued boat-holders at the site, beside Watermans arts centre, with notices telling them they are contravening planning regulations.
But many of the boat dwellers, who claim some of the vessels have been there since the '70s, are believed to be taking legal advice to help them fight the marina plans.
Paul Mendoza, a photographer who says he grew up in Brentford and has lived beside Watermans Park since 1997, said: "The council's claiming our moorings represent inappropriate development. If that's the case how can it say it wants to create moorings of its own for 26 boats here?
"We're concerned that the footprint of a much-loved, well-used park would be reduced by this development, which would be of no benefit to the community."
Mr Mendoza added that the marina plans were part of the over-development of Brentford, which was making the area unaffordable for ordinary people.
Hounslow Council claims the land in question is poorly used and improvements to the remainder of the park would be carried out in return for its loss.
The council has previously claimed the existing boats are unsightly and their occupiers do not pay council tax which is unfair on other residents, though boat dwellers have told getwestlondon they would be willing to pay council tax.
A public exhibition on the £5.45m marina plans, including the car parking spaces, was held last autumn and the council said two thirds of responses received were in support of the proposals.
A council spokeswoman added: "The land will be appropriated after planning consent, subject to consultation. The river frontage and riverbed will need to be cleared and cleaned to enable the development to take place.
"LBH and the Port of London Authority have made boat-holders aware of the plans and the need for them to source alternative moorings should the development be approved. Two further public consultations are also planned in June."
getwestlondon understands most boat-holders are planning to stay and fight the plans, but a minority are in discussions with the council about alternative moorings.
Should the marina receive planning permission, it is scheduled to be completed by late 2017.