It's a big day for the creatures which live along the UK's 11,000-mile coast.
The government is releasing details of how it plans to protect our sealife, which is under threat from fishing, pollution and climate change.
But conservationists want even tougher measures brought in to stop our sea and its creatures getting damaged.
The Marine Conservation Society is calling for a massive network of off-shore reserves where any harmful activity is banned.
Members are marching with other campaign groups to the Houses of Parliament in London, to deliver a petition backing their demands.
Conservationists want to protect sealife
Some people oppose them because many people earn their living through fishing and other sea-based jobs.
Newsround asked kids what they thought about the issue.
Jika said: "People should stop putting giant fishing nets at the bottom of the sea because it will destroy the coral."
Nerissa added: "If I was in charge, I would look after the sea and ban people from throwing rubbish into it."