"Found the perfect place!... Storm forecast for tonight! And this gentleman's kindly agreed to lend us his boat!"
-- Vernon Dursley, gleefully (PS3)
It’s hard to say exactly which sea the Hut-on-the-Rock rests in, as really any body of water other than the English Channel would have been quite a drive from Little Whinging. However, the Hut-on-the-Rock is indeed where Vernon Dursley drove his family in an attempt to escape the Hogwarts letters that were coming for Harry (PS3) on the mistaken belief that wizards can’t cross water (Pm).
It was there that Hagrid finally caught up with Harry and the Dursleys. After banging down the door, destroying Vernon’s gun, and giving Dudley a tail, he told Harry – for the first time – the story of his life, of Voldemort, and of the Wizarding World (PS4).
Inside the hut it smells strongly of seaweed. There are gaps in the wooden walls and the windows are filthy. The hut does have a fireplace in one of its two rooms. Several moldy blankets can be found in the hut, along with a moth-eaten sofa. “The Rock” is really a tiny island, as the only way to get to it was by rowboat. Well, the only Muggle way, anyhow (PS4).
One fan theory has it that the hut on the rock must be off the western coast of Britain, perhaps in the Bristol Channel, because Harry takes the train from Paddington to go "back to the Dursleys." This assumes that the Dursleys are still in the vicinity of the hut on the rock at the end of the day which Harry spends in London. Much more probable is that the Dursleys traveled back to Little Whinging during that day, which makes this clue indicative of the location of Privet Drive rather than of the hut.