Pretty simple wherever you are on the sea you're in someone's claimed waters.
This can still allow for disputed waters claimed by multiple nations or cooperative waters like say the Channel where two nations control the waters together.
Of course for most of the world your chance of being caught flaunting laws are very slim but not none
Could you give an example of what sort of dynamic might lead to such a thing? Obviously any maniacal autocrat (or vainglorious republic for that matter) can go around making grandiose claims, but why should it become the norm for enemy, or rival, powers to recognize each other's claims?
In comparison consider the Age of Exploration practice of European nations to "claim" islands and coastlines their navigators "discovered." If a French explorer claimed an island, would the British ignore this, put in their own claim and name, and would possession of the island in a later era be a simple matter of who got there first with the most force--or did claims turn out to have legal force and effectiveness? Why would they? How could they?
Often enough, one European power would violate another's claim to be sure. The question is, why were they ever respected, and I think the answer is that in many cases a region would be thinly held by a handful of Europeans, but there was broad room for both to profit if they stayed out of each other's way, whereas if they were constantly plotting to seize each other's bases the whole margin for profit would be eaten up in fortification attempts. It mades some sense, some of the time, to concede that some rival power did indeed have possession of some colonial land if at the same time that dangerous rival would similarly concede your own possession of your colonial outposts. The European system of claiming lands for this or that kingdom was an extension of this logic, in water where the rival assets fluctuated between a handful of ships and absolute zero. What good would it do a captain like Cook to "discover" and claim an island when it will all be ignored the second he sails away? Without some agreement that what is mapped is claimed, unknown waters like the Pacific would be near perfect anarchies.
To an extent too, the power that first discovers a land has an inherent advantage after all--they
were the first Europeans to get there and survive to tell the tale back in Europe, implying a second and Nth ship from the same home port could get there just as well, while it is up in the air whether some rival European power could find and supply that same base.
Now I ask you to provide some kind of reasoning that provides a basis for the collective network of European seafaring nations to come to agreement that all waters of all the world all belong to some shore-bound power, and on what basis to partition them?
It should be noted that OTL, it was commonly recognized that a nation had coastal waters--and these were held to be the very limited band in which land-based power multiplied the grip on generic sea power. For instance through the 19th century it was held to be "within cannon shot" which is to say within range of possible cannon-armed fortresses on land, which would tip the balance in a battle obviously. Such national waters were obviously extremely short range.
You
will of course find historic OTL precedent for sweeping claims of vast areas of ocean exclusive to one power. By encircling the entire Mediterranean with land power, the Romans were effectively able to claim the Med as "Mare Nostrum." Portugal and Spain were able for a while to claim, among themselves and European powers compelled or with an interest to honor papal decree, the majority of the Earth's waters as belonging to them. Of course these claims had meaning only as far as either kingdom could project power, and mainly versus other European powers, as long as they could control the routes to the distant seas claimed. By the early 17th century powers such as the Dutch East India Company could defy either kingdom with near impunity and the evolution of the doctrine that nations could claim control only over that portion of ocean they could effectively control, practically interpreted as the then prevailing range of cannon fire--3 nautical miles. Since then the majority of states have shifted to a 12 NM territorial claim, with a few claiming more and with wider zones over which nations are held to be entitled to control of limited categories--the USA claims the right to regulate the resources found on our continental shelf which extends in some cases beyond even the 200 mile economic interest zone widely recognized.
So there are two opportunities I suppose to get the entire World ocean under some flag or other:
1) Triumphant early maritime empire claiming the whole damn world. You might note that even Portugal and Spain lacked quite the chutzpah to claim the traditionally freely navigated waters around Europe itself, leaving those to be quarreled over by powers like Denmark and England...but one supposes that had the Hapsburg system evolved to go from strength to strength they would gradually crush rival European bases, secure monopoly relations with non-European powers, and by and by having made good their claims to utterly control their "outer" seas, they (presumably at some point irrevocably merged into one system) could backfile as it were and seize control of the Med and Atlantic waters as well. Technically meets your spec--all waters in the world belong to one monarch.
2) long after Grotius's Mare Liberum principles are broadly established and the principle of nations claiming only what is within land artillery range widely accepted, changed conditions lead to a total partitioning of the world.
Say we have a situation not unlike the outcome of WWII OTL, but without as severe a threat of an immediate WWIII following and with a more cynical partitioning of the world into spheres of interest. Actually Winston Churchill had a model of the post-war world regime along such lines; USA, British Empire, Soviet Union and I forget if there would have been a fourth power at all would between them divide the world into spheres of influence, and govern with little oversight from the others within that sphere, and the powers would confer to arrive at joint global policy together. In such a scheme, each hegemonic power would in effect claim the formerly international waters as its own to police and regulate.