I don't have any experience about this, but since we normally use
water containers in buildings and the convective currents are not
noticeable, I think you can define a new material with the following
properties:
Thermal Conductivity: 0.58 [W/mK]
Density: 1000 [kg/m3]
Specific Heat: 4.18 [kJ/kgK]
Farhang