I didn't say that Britain "ruled the waves" during the SYW. The French navy was not equal match to the RN in the war, but was able to challenge the British seriously. Indeed, seaborne invasion was considered a serious possibility at least up to Trafalgar by the British. The point is that the British had the leisure to plan accordingly, namely they had not to worry about a large navy and a large army at same time, which was France's problem (and to a point, Spain's problem too). OTOH, Prussia could focus all its energies on the army (and did so astonishingly well).
A joint Spanish-French naval force could match the British (as it did in the ARW, I believe). However, Austria and Sardinia would be able to contribute importantly to such a force.
As for the North American theatre, I am under the impression that it is important mostly in hindsight, and that both players saw it as a sideshow at the time. Of course, British naval superiority, while not uncontested, was still critical in British success in that sector.
By the way, had the Franco-Austro-Russian coalition stomped Prussia as planned, France would have been free to focus on the seas; in that case, balance of naval forces and, therefore, the situation in North America would have been less favorable to the British I think.
