He suggests sailors couldn’t afford all the alcohol they were given when out of the Navy so stayed on ships for their daily rum ration. I think they just drunk grog to get through the terrible conditions and because it didn’t spoil.
We’re watching master and commander and trying to work out why men would go to work on these ships in such horrible conditions!
More may of course be said, but you may wish to check out these past answers by /u/jschooltiger which discuss alcohol provision in the Royal Navy: