Television chef Hugh Fearnley-Whittingstall said it was "vital" the Government did more to protect UK seas as he led hundreds of campaigners in a march to Westminster.
The Westcountry-based presenter has urged ministers to increase the number of "marine conservation zones" after more than £8 million was spent on identifying 127 areas where dolphins, seahorses and other rare species most need protection.
The Department for Environment, Food and Rural Affairs has so far managed to set out plans to create 31 sites by the end of this year, while the majority of MCZs have no timetable.
Speaking outside the Houses of Parliament, Mr Fearnley-Whittingstall, whose River Cottage food business is based at Axminster, East Devon, said: "We're all here today because we believe in the sea. We know it has an incredible power to enhance our lives.
"There's a public consultation going on right now over the proposals of 127 new marine reserves around the UK.
"So far we're a little concerned that the Government are only going to look at 31 of those in the coming year, and at this point they haven't given us a time frame for the rest.
"This is the sort of opportunity that may not come again. We might not have such a vital and appropriate time frame as we've got right now to make real changes.
"If we leave it too much later, too much damage will have been done. It will be hard for a lot of the areas to recover.
"What's particularly worrying is there are 33 areas that scientists have assessed as being in urgent need of protection and recovery that have not been included in the areas to be considered."
Mr Fearnley-Whittingstall joined forces with the Marine Conservation Society (MCS), Sea Life, the British Sub Aqua Club (BSAC) at the rally.
BSAC divers arrived in inflatable boats at Westminster Bridge, while Sea Life visitors have signed an estimated 40,000 postcards in support of MCZs.
Dr Jean-Luc Solandt, biodiversity policy officer at the Marine Conservation Society, said: "We need to protect our seas now. More than a million people took part in the process that resulted in the list of 127 recommended MCZs. It cost the British taxpayer in excess of £8 million.
"Government must make a firm commitment to designate these now, and the rest of the 127 zones in 2014."
Sea Life senior marine biologist Rob Hicks said: "The evidence of dwindling catches, shrinking average sizes of those fish that are caught and disappearance of species like the undulate ray is sufficient in our view to justify action now. It may well be too late in four, five or ten year's time."