The Royal Navy ruled the sea for 200 PLUS years.
How come it hasn't done so since after World War 2?
The powers to be allowed it to get smaller and smaller.
Not only was the British Empire on the ropes financially, very quickly after WWII the Empire became a Commonwealth, losing most of its colonies for a host of reasons.
No far flung colonies and protectorates = no need for a global reach Navy, particularly when the USA was more than happy to perform the duty of protecting international shipping lanes and ensuring the free flow of trade over the waves.