A Bloody Mary is a consumable item, available in Risen 2 and 3. It increases a character's regeneration by 100. This means that 100 points of health will be restored over time, rather than instantly.
Certain merchants will sell individual bottles of Bloody Mary, such as Hawkins or Spencer. They will charge 75 gold if no items give the player a discount. They generally do not carry many bottles.
In Risen 2, it is also possible to get a recipe that will allow you to brew your own Bloody Marys in the future. This recipe is obtained by completing the 10 Bloody Roots quest.
There is no recipe available to the Son of Steelbeard in Risen 3.