Basically, your challenge is to prevent Germany from being the dominant naval power from the 15th into the early 20th century, thereby preventing the trope that “Germania rules the waves” as it is commonly said here on AH dot com. Also, what other country could have ruled the waves? Is France or Russia possible? Maybe Scotland or Ireland? How different would the world look with another one of the countries in Europe ruling the waves?
The whole idea is slightly too much on a fantastic side even for the AH but let's try....
Probably the critical POD would be Wallenstein's dismissal somewhere around 1630 in an unlikely scenario of Emperor Ferdinand II caving to the demands of the German electors. Who, in his right mind would dismiss his most successful general and creator of a powerful imperial navy (which just helped to capture Stralsund and forced Denmark to beg for peace) in an exchange of a vague promise to vote for his son in the next imperial elections? To start with, it is not like Ferdinand was already dying and then an assumption that Ferdinand was too stupid to figure out that with 80 - 100,000 of his troops occupying Germany, the chances of electors NOT voting for his son are quite low.
But if we chose to ignore all these glaring improbabilities, then you are getting your scenario:
1. Wallenstein, "
Admiral of the
North and
Baltic Seas" and supreme commander of the imperial armiesis dismissed.
2. Imperial help to the Commonwealth did not happen and Gustav Adolph is victorious with Livonia in his possession.
3. Imperial Navy keeps rotting in Wismar and Rostock (in the former Wallenstein's Duchies of Mecklenburg which he left after dismissal).
4. Command of the Imperial army is transferred to Tilly.
5. With the Imperial navy being inactive, Gustav Adolph lands in the Northern Germany and one of the 1st thing he does is destruction of the surviving imperial ships (formally done by the dukes of Mecklenburg whom he restored).
6. A much longer war with a possibility of escalation from an obscure regional conflict all the way to a major European War with the bizarre combinations of its participants. Can you imagine it is continuing for, say, 30 years (the 1st unrealistic number that came to my mind) with, say, Catholic France siding with a Protestant Sweden to fight against the Catholic Hapsburgs? What would be the chances for such a nonsense?
7. Among the numerous domino effects will be a much later unification of Germany by the Hapsburgs (somewhere in the late XIX).
To answer your question, about an early extermination of the imperial navy and potential candidates to the naval domination:
(a) The Netherlands - well, of course, they had the merchant and fishing fleet but they are tiny and why do they need a navy? To help catching more herring?
(b) Britain (of course, I'm just kidding: probability is just a notch or two higher than for Curland) - building a powerful navy just based on the dwindling incomes from selling a low quality wool, really?
(c) France, of course, would be a reasonably serious candidate but its national addiction to the civil wars and revolutions, which started in the mid-XVII (War of the Red Heels, the 1st of the "Fashion Wars" conducted by Louis XIV) and ended only in the early XX with so-called "2nd Restaurant War" (the French were always taking their cuisine quite seriously and China refusal to open then chain of the French restaurants resulted in a nasty affair in which unknown numbers of Chinese had been killed, local production of the noodles was completely destroyed and defeated Chinese government was forced to outlaw usage of the chopsticks) closely followed by "Little Black Dress" coup (which guaranteed French domination of the fashions world) that brought to power Chanel dynasty and finally stabilize the country, did not leave France with too much time or energy for pursuing the follies like "the waves domination". Really, why do you need rule over the waves (so you are ruling them and what you are getting as a reward? A fish?) when you are ruling the eating places and boutiques all over the world? Well, of course having a navy proved to be useful in the 2nd Restaurant War but this was just a useful tool built for this specific occasion and rarely used afterwards.
(d) Russia would do it if not its problem with the "waves": it is rather difficult to dominate them while having access only to the Arctic Ocean, Pacific Ocean and Caspian Sea. Arctic waters are under ice most of the year and until construction of the powerful icebreakers in mid-XX (out of your time frame) not very useful in the terms of a navigation. Pacific coast is much better but it was practically undeveloped until the late XIX. Of course, there was a victory of a newly built Russian navy over the newly built Japanese navy during the Russian-Japanese War of 1904-5 but you probably agree that a naval victory at La Pérouse Strait and even following Russian occupation of Hokkaido hardly amount to the "rule of the waves". IIRC, in the XVIII century there was a bizarre project of creating a fleet on the Baltic Sea with its base on the Swedish territory. Sweden (always friendly to Russia after Peace of Stolbovo) would give a permission to build a village on its cost (called "Potemkin village" after the author of project) and the Russian warships built on the Swedish wharves would use it as an official base. The whole project was just a "prestige" thingy and had been abandoned in a favor of conquest of Manchuria.
(e) Scotland is a little bit of a surprising item on your list. Of course, it completely broke union with England during the ECW (execution of one Scottish monarch could be tolerated as a misunderstanding but two amounted to an evil pattern) and even built a beautiful wall along the border to stop the English immigration. Without being burdened by a need to support that big backward territory on the south, they had enough money for a naval buildup. However, they did not need a powerful navy to enforce exports of the scotch upon the rest of the civilized world: everybody was and is buying it quite willingly. With their reputation for being stingy .... oops ... frugal, why would they waste money on something absolutely unnecessary?
(f) Ireland. IIRC, it was occupied by the Brits until the end of your time frame and since then they keep celebrating their liberation (or did they already stop?)
(g) Denmark had, for a while, a powerful navy but most of it was destroyed by Wallenstein and the rest sunk by the Swedes.
(h) Sweden would be a potential candidate but they never were truly interested in anything but getting their custom dues from the grain trade through the Baltic ports.