If the environment consists of different particles than the single molecule, the situation is similar, even if the molecule is as heavy or heavier than the particles in the environment.
But a water molecule surrounded by water is different as the different water molecules are indistinguishable. (A practical question is: How does one remember which molecule was singled out, as all are indistinguishable?)
In any case, the answer is here given by the 1-particle reduced density operator of statistical mechanics (if the water molecule is considered as asingle particle). See. e.g., Chapter 7F of Reichl's book on statistical Physics.