AskHistorians

Found an article here:

Norton, Louis A. "RUM: The Spirit of the Sea."Naval History, vol. 31, no. 2, Apr., 2017, pp. 20-23.

On higher alcohol content for the navy:

The naval tradition of consuming alcohol at sea has a long history. In the beginning, this attempt to reduce some of the boredom of shipboard duty was a public-health measure. Potable water stowed on board a ship at sea for any length of time became stagnant, with algae blooming within the storage casks, so mariners quenched their thirst with beer. Its alcoholic content and hops helped control the growth of microorganisms, and a daily allowance of beer helped take sailors' minds off shipboard difficulties. In more southerly latitudes, however, beer would spoil before the vessel reached her destination. Officers often sailed with a stash of whiskey, and it was noted that this spirit, with a higher alcoholic content, did not have a spoilage problem.

On rum and percentage, it seems it was much higher!

During the 17th century, an inexpensive, plentiful, and nonperishable spirit became available for the lower ranks on board ships in the Caribbean. As European countries colonized the islands, the newcomers established sugarcane plantations. Local sugarcane could be made into molasses that when fermented and distilled yielded a 140 proof (70 percent) alcoholic beverage that became known as rum. Before long it began to be used as an economical reward for the efforts of sailors on board naval vessels sent to defend the island outposts from pirates and invasion from hostile nations.

Rum and the navy

When demand for sugar increased, sugarcane plantations spread rapidly, and the planters found themselves with more rum than they could sell locally. Because a large inventory of the spirit in Caribbean warehouses was an open invitation for trouble from pirates, plantation managers sought and found another ready market-Royal Navy pursers. Ironically, British colonies initiallly were forbidden from exporting sugarcane distillates, but the Royal Navy's purchase of spirits as a victual was deemed permissible because it enhanced the life of the king's sailors as well as provided a windfall for the plantation owners. In 1687 the Royal Navy officially adopted rum from British Caribbean islands as part of a crew's daily ration. This act was the genesis of a naval tradition that has lasted almost 300 years.

I think this is much clearer for the navy and also shows I may be wrong about gin. They were drinking either 70% rum or grog. It seems as time went on the navy would weaken the rum so that the sailors weren’t drunk all the time.