The difference between a hypothesis and theory is that the hypothesis is based on observations, but doesn't have a solid proof, while the theory is based on solid proof.
So in order for the scientist to make his hypothesis into theory about the connection of all the Earth's oceans, will need to provide a proof. This proof can be obtained by a sea traveling that will be well documented, and during the travel the scientist to manage to sail through all of the Earth's oceans without having any physical obstacle between any of them.
