The principle of oceans being international waters and sea travel being free for every state, as well as it being disallowed for oceanic/sea waters to be annexed is strongly established and universally accepted. However it hasn't been legaly sanctioned by international law for a very long time, nor was it taken for granted: indeed Roman Law concidered seas to be annexable (mare nostrum) and one aspect of the English - Spanish naval competition in the XVI - XVII century was over this issue. The Spanish supported the mare nostrum concept, the English opposed it and fought against it. Even today seas aren't completly free, with some areas going beyond traditional 12 nautilic miles territorial waters being considered as those (Russia and the White Sea for example)
So what if the freedom of the seas principle is not established? Perhaps Spain wins the competition with England, Grotiuses works don't become as popular as OTL, or Roman Law is introduced to Europe more strictly, perhaps some time during the enlightment (the enlighted ones looked towards the antique times and Roman Law as inspiration). How would that affect international relation, trade, law?