The suggestion was floated in the wake of the EU referendum in the UK by Geert Bourgeois, the Minister-President of Flanders.
Bourgeois, a member of the Flemish nationalist N-VA party, made his proposal this week on a visit to the Flemish Institute for the Sea in Ostend.
He said that taking into account that Britain is a major export market for Flanders, he was thinking of ways to avoid a hard landing, and to go for a so-called 'soft' Brexit.
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He says there is a growing consensus in EU capitals that it would be a mistake to "punish" the UK for voting to quit the EU.
The idea of North Sea Union was actually first proposed by the German 'Land' of Bremen six years ago, but has suddenly become topical again with the Brexit vote. Denmark, Germany, the Netherlands, France, Sweden, and Norway would be the obvious members.
Under such plans, the EU would promote an integrated sphere for offshore energy and marine research, and as well as better grid network with interconnectors to drive down costs and boost back-up power. It would have a defence and security component.
"I am not proposing a new 'EU'. My idea is a light structure on an intergovernmental basis, like the Union for the Mediterranean (UfM).
"There are so many areas in which we can work together on the enormous potential of 'blue industry' in the oceans, whether it is pharmaceutical sector, health, or food production," he said.
There is already a North Sea Commission made up of regions with a stake in the marine economy , currently under a Swedish President.
The UfM was originally an attempt by France to carve out a French-led zone of southern European states with distinct ties to north Africa, and was seen as a move to counter German dominance.
The UfM includes 28 countries bordering the Mediterranean sea, from Europe, the Middle East and Africa.
So far, though, it has met with mixed success.
Andrew Duff, a former leading UK MEP and now Visiting Fellow at the Brussels-based think tank, European Policy Centre, is unsure about a North Sea Union.
On Thursday, the Liberal politician told this website, "Anything that takes the Union for the Mediterranean as its starting point deserves to fail. In any case, the UfM is not a trade agreement. Nor can a trade agreement be reached outside the legal framework of the EU."
Duff, a constitutional expert, added, "However, as the UK has decided to escape from the community method, there are plenty of intergovernmental association agreements on a bilateral or multilateral basis that will be tried.
"Trade and tourism promotion, for example, might work. Even if the North sea regions just create another talking shop, that will do no harm."
More reaction came from UK Tory MEP Charles Tannock, who told the Parliament magazine, "I welcome any fresh thinking to soften the blow of Brexit, which I regard as deeply regrettable and will inevitably reduce UK prosperity and influence in European and global affairs.
"I think it will be legally tricky for the UK government to negotiate directly with a regional government and without the support of Belgium's Prime Minister Charles Michel of the national federal government in Brussels, but it is worth pointing out the UK Conservative party led by Prime Minister Theresa May has excellent bilateral relations through the NV-A party being together with UK Conservatives in the ECR group which facilitates dialogue.
"However this North Sea Union appears mainly as an intergovernmental talking shop which does nothing to guarantee UK unfettered access to the EU single market which is my priority today safeguard our economic interests."
A Conservative delegation spokesperson said, "We welcome all positive suggestions as the UK looks to forge a new, mutually beneficial relationship with the EU. Bourgeois is right to point out that it will be in the interests of both sides to continue to co-operate closely across a wide range of issues."
Elsewhere, the man responsible for the North sea in the Belgian federal government, Philippe De Backer, has rejected the idea.
The Flemish liberal told the regional West-Flemish TV station WTV: "He (Bourgeois) is taking it one step too far. There is a cooperation frame at present, I really don't want any new structures. Then you need more people, more civil servants, more money."
"Many of the proposals Bourgeois is making, are made with good intentions, but already exist on a federal level today. We should focus on cooperating on this level, confirming and developing the existing structures."