Asked by: Miranda Caddy, London
The amount of water you lose through urinating is controlled by the Anti Diuretic Hormone (AHD, also called vasopressin). ADH causes your kidneys to extract excess water from your urine so that it can be recycled back into your body. Without any ADH at all, your kidneys would dump about 10 litres of water a day into your bladder; ADH reduces this to one or two litres.
Alcohol increases the amount you urinate because it suppresses the production of ADH. A sufficiently alcoholic drink can suppress ADH to the point where your kidneys actually excrete more water than the volume of the drink itself, and so there's a net dehydrating effect. But the concentration of alcohol required for this increases as you get thirstier. If you just drank wine on your desert island, you would initially lose more water than you gain from wine, but as your body became more dehydrated, it would produce more ADH to compensate and you'd eventually reach an equilibrium point.
For the 13 per cent alcohol content of most wines, that equilibrium point would still leave you badly dehydrated (not to mention hopelessly drunk), but it should prevent you from dying of thirst. If a six-pack of beer (5 per cent alcohol) washed up next to the wine, that would be a better choice.