I'd imagine it was and still is more economical to transport large quantities at a higher concentration and diluting later if desired.
We don't actually know at what proof rum of the period I study was, but most guesses put it into the range of 100-110 proof. After 1816, when an accurate hydrometer was patented, proofs started to be standardized and official "Navy proof" of 109, or 54.5% alcohol, was enacted for victualling. In terms of drinking it watered down, officers may have done so in punch or similar mixed drinks, but sailors were only supposed to have their ration amount of spirits (there was an illicit trade in spirits aboard ship, but that's a separate issue).
I've written some more about alcohol in the age of sail if it's of interest: