What exactly makes a "good natural harbor?"

St. George’s, Grenada, small, but a welcome sight after a long sail. On the leeward side of the island, too; although when a hurricane comes through even that’s not going to help much.

Sydney Harbour is in a flooded valley with steepish sides, so you’ve got good depth, and there’s a nifty kink in the thing to shield the port from any swell. It only gets bumpy once you’re approaching the open ocean (and then not for long if you’re just going past).