TV chef Hugh Fearnley-Whittingstall said it was "vital" that the Government did more to protect UK seas as he led hundreds of campaigners in a march to Westminster in London.
The presenter has urged ministers to increase the number of "marine conservation zones" (MCZs) after more than £8 million was spent on identifying 127 areas where dolphins, seahorses and other rare species most need protection - many zones covering the south east and southern shorelines.
The Department for Environment, Food and Rural Affairs (Defra) 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 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.