I'm happy you brought that up. It's more than a simple declaration of intent.
As it so happens, a decision was reached in 1933 in the matter of a dispute between Norway and Denmark over the sovereignty of the eastern part of Greenland. The court held that when a head of State or a representative speaks on behalf of his government, even if only to make a simple declaration, the government is bound by the declaration, even if it is unilateral. In this case, the declaration was made by five countries, following a special conference the purpose of which was to determine the degree to which the five States could agree.
Some argued that the Arctic Ocean was a very unique body of water and that special legislation was needed, to put the issue into Canadian legal terms. However, the Arctic Ocean is subject to the United Nations Law of the Sea Convention.