You will need to look at BS IEC 60479 series if you are looking at specific information other than dry location.
In BS 7671 "wet location" is dealt with by special requirements in Part 7. Product standards take other approaches (including ignoring wet condition if the product is not anticipated to be used in that condition).
As Graham above has already stated various wet locations are listed in Part 7 as they are a higher risk so need additional consideration. For that reason you will see they need 30mA Additional Protection.
If you go back far enough in old regs, the maximum accessible touch voltage that is normally 50V, used to be set to 25 in damp or conductive locations, suggesting an assumed halving of the body resistance.
Of course in reality getting a person wet only alters their surface resistance, the moisture levels of internal organs is not significantly affected. Therefore the effect of being coated in sweat or sea water is more akin to a dry contact over a larger contact area, but as the entry and exit wounds indicate, most of the heat, and so most of the resistance, is where the current path breaks the epidermis.
Once charring starts the resistance drops sharply, water may reduce this surface burning effect by providing cooling and improving the contact, at the penalty of a higher initial current. None of this is good.
You can usually half the body resistance, which, in effect corrolates to half the shock voltage when the body is wet, as Mike has stated. These safety measures are usually achieved by supplementary bonding and/or RCDs, reg 415.
RCDs seem to be the preferred method of protection today. See this link below.
