I would take ‘high seas’ as just a figure of speech and not try to distinguish coastal from deep sea casualties. After all a lot of merchant ships were wrecked by being driven ashore, IOW technically approximately zero miles out at sea when actually wrecked.
On total number given OP criteria “In the sense that countries as we know of now have existed in the last few hundred years”, I agree that’s very likely Britain.
As far as losing people at a single war at sea, it’s probably Japan which as outlined above lost on the order of half a million people at sea in the Pacific War (300-400k IJN personnel most though not all at sea, another 200k+ Army and civilian personnel lost on merchant ships). However the linked book quotes the loss of British merchant seaman just to accidents (shipwrecks and individual accidents, somebody fell from the rigging or into a hold, etc) in 1881 as over 3,000, ie not including disease. The number of British seafarers might have been smaller in earlier centuries but ships had gotten quote a bit safer by the late 19th century than previous several centuries. Several centuries of 3k per year would handily exceed the heavy Japanese toll in WWII (plus gradual losses in the smaller Japanese merchant fleet). The Spanish, Portugese and Dutch fleets were significant and often pursued extremely dangerous voyages, those to the Far East ca. 16-17th century had loss rates as high as 20%. But I’d guess the larger scale of the British fleet and particularly later on in the period of dangerous seafaring would put it first. Which country has the most merchant seafarers now (which is the Philippines, by far) is irrelevant to the question because even though merchant seafaring is still a relatively dangerous job by modern standards, the death rate now is negligible compared to what it was in the age of sail. Likewise peacetime losses at sea in modern navies are at a very low rate compared to the age of sail. Seafaring Labour: The Merchant Marine of Atlantic Canada, 1820-1914 - Eric W. Sager - Google Books
Ah, but, do those countries fulfill the OP’s concept? At the time you’re speaking of they were geocultural concepts, not fully-independent domains; both Portugal and (parts of) Holland were borne by the same heads which bore the Spanish crowns, and Portugal was even part of the original concept of Hesperia (Spain).
For most of its history, “Spain” has been a geographical concept (the Iberian peninsula), later a geopolitical/geocultural concept (those parts of the Iberian peninsula where Spanish was the main language used for government purposes, plus their colonies). “Portugal” was part of Spain as a geographical concept at the time of its birth as a kingdom, but eventually became a different geopolitical/geocultural concept; it was part of Felipe II’s domains, but re-separated from the multiple states which would eventually either gain independence or form the current Kingdom of Spain (officially formed in the 19th century, and since been two Republics and one Dictatorship as well as three times a Kingdom) at Felipe’s death.
During the time of the Great Naval Discoveries, “the Netherlands” was several colored blobs on any map: independent or semi-independent cities, part of the Holy Roman Empire (aka “Germany”), part of France (there is a whole chunk of modern France which is called French Flanders or the French Lowlands), one of the crowns held by the Spaniards (modern-day Belgium, founded in 1830, corresponds roughly to the so-called “Spanish low countries”, but notice that the origin of these was in Burgundy, which is now France), source of the British crown… those blobs blobbed about a lot. Take ten maps drawn on ten different decades and you’ll have different borders and different connections to other realms.
That’s what I’m asking for, a clarification. The OP seems to have been very sure of what he meant, but I don’t think he was very clear on the meaning of several of the terms he used. He mixes “going to sea for their king and country” with “tradesmen”…
