Kiwi Conservation Club member Dannie Hawkins, of Dunedin, said his family enjoyed visiting marine reserves around New Zealand, including at Ulva and Goat Islands.
The financial planner said greater marine protection in the South was a "no-brainer".
The forum was calling for submissions on proposals for 20 sites for possible inclusion in a network of marine protected areas.
The reserves stretch from north of the Pareora River, near Timaru, to Waipapa Point in southeast Southland.
Mr Hawkins was "surprised" how few marine reserves had been proposed.
"What they are proposing is a bare minimum."
Forest & Bird chief executive officer Kevin Hague published an open letter to forum chairwoman Maree Baker-Galloway in the Otago Daily Times on Wednesday.
The letter mentioned comments Ms Baker-Galloway made in the Otago Daily Times on November 21.
Mr Hague said the comments appeared to suggest the size and number of marine reserves proposed would probably be reduced during the consultation period.
A scientific consensus revealed at least 30% of the ocean must be protected in "no-take" marine reserves to ensure "genuine" marine sustainability.
The forum was proposing a "mere 5.3%" of the ocean to be reserve, he said.
"This is a hopelessly inadequate fraction of what is required," Mr Hague said.
Conservation land in New Zealand covered 30% of the nation but less than 1% of the ocean surrounding New Zealand was protected.
Forest & Bird was calling for public submissions in support of reserves to protect marine species, including New Zealand sea lions, albatrosses, penguins and Hector’s dolphins.
"The ocean is in crisis. Our attitudes and practices need to change, and fast. Marine ecosystems are being devastated by over-fishing and by-catch, mineral exploitation, climate change and scariest of all — the overwhelming belief that everything is fine."
Ms Baker-Galloway said the wording by Forest & Bird in the letter differed from the comments she made on November 21.
"They’ve made a statement that doesn’t reflect what was said."
The objective of the submission process was to obtain knowledge to help the forum make an informed decision with an "open mind".
"The more information people can give us about what they know and what they think, the more the forum will be basing its decision on the best available information."
She hoped Forest & Bird would lodge a detailed submission including the consensus data, as the forum had not been given the information yet.
The extra information would make the final decision "stronger and more dependable" as it would be based on the best information available.
"If people sit back and don’t tell us what they think then the decision will reflect that," Ms Baker-Galloway said.
More sites would not be added to the proposal because it would create an "unfair" process — people would not get a chance to have another say before the forum had to present its recommendations to the Government in April.
"The process has to be fair within that timeframe."
The submission period closes at 5pm on December 20.
